As the United States’ quagmire in Iran makes clear, wars are almost always much harder to stop than they are to start. Over the past 80 years, less than 20 percent of the roughly 300 wars tracked by the Uppsala Conflict Data Program ended in a clear victory by one side. A slightly smaller percentage ended in negotiated settlement; more than 60 percent had no formal ending. And the longer a conflict continues, the harder it becomes to broker a settlement. In wars fought between 1945 and 1989, another study found, mediation succeeded 37 percent of the time when it began within the first one to three months of fighting but only 19 percent of the time when negotiations began a year or more into a war.
The United States’ two-decade war in Afghanistan and decade-plus fighting wars in Iraq were no exception. In the wake of those conflicts, Washington has tried to avoid getting ensnared in so-called endless wars by avoiding deploying boots on the ground. In some cases, that means the United States relies primarily on airpower, as it has in Iran—an approach that comes with its own military challenges. But in many others, instead of participating directly, the United States is equipping close allies to fight shared threats and joint adversaries. Between 2010 and 2025, Washington more than tripled its annual security assistance for allies, from $15 billion to $46 billion, and increased its intelligence support and arm sales.
The United States has backed several of its partners’ wars in recent years. In 2015, Washington began coordinating U.S. intelligence and military support for the Saudi-led coalition fighting the Houthis in Yemen. Since 2022, it has supported Ukraine in its defense against Russia’s invasion. And the United States has aided Israel throughout the war in Gaza following Hamas’s attacks on October 7, 2023. In all three campaigns, Washington supplied or sold the overwhelming share of the military hardware, intelligence support, training, and diplomatic backing. Yet its efforts had mixed results. The United States’ support greatly enhanced the military prospects of allies, but in every case, Washington found itself unable to shape how these wars were fought, and more critically, when and how they might end.
Supporting an ally’s war effort is often in the United States’ interest. But if Washington wants to participate in conflicts from afar, it needs a better strategy for maintaining influence over the primary combatants. It needs to enter with a clear idea of the war’s aims and how to achieve them. It needs to secure its partner’s commitment to the same objectives, to be honest with that partner about how far U.S. backing will go, and to set up mechanisms to ensure that the partner’s military adheres to American standards of conduct in battle. Ultimately, continuing to support allies’ wars without a shared strategy and without clear conditions for aid will not deliver the United States the advantages it seeks. It will only lead to more tension with close allies and more conflicts that don’t end.
THE LEVERAGE TRAP
Together with my colleagues at the Brookings Institution, I spent the past year interviewing more than 30 senior American officials, reviewing hundreds of public statements and documents, and consulting foreign officials to put together a comprehensive picture of how Washington engaged its partners during recent conflicts abroad and how those American efforts were received. What we found was that the United States tends to encounter many of the same problems when it supports its allies’ wars, even when the specifics of each situation vary quite a lot.
At the start of a conflict, Washington has much more power to shape a partner’s decisions than it does later in the war. When allies come under attack, they feel stunned and insecure, and they tend to seek Washington’s support in the belief that they will need U.S. military power to win their fight. Yet the U.S. government consistently fails to use its leverage early on. By the time it tries to push its partners to change operational designs, adjust tactics to minimize civilian harm, or come to the negotiating table, its influence has already eroded.
This failure can partly be attributed to the enormous political and moral pressure Washington feels to offer unconditional support when a close ally is attacked, as Ukraine was in 2022 or Israel was in 2023. Demanding strategic concessions from a beleaguered ally can feel like a betrayal. But holding back defers crucial conversations about goals, methods, and the limits of American aid.
Wars are almost always much harder to stop than they are to start.
Consider Ukraine. A few months into the war, the Biden administration publicly tied Ukrainian battlefield gains in May 2022 to the objective of improving Ukraine’s negotiating position with Russia, implying that Washington might expect Kyiv to start discussing peace terms before it could regain all of its land. But it is not clear that Ukrainian leaders understood or accepted the American strategy. As one U.S. official recalled, “It’s not that we didn’t take the time to think through the mission’s objectives—we did.” But Washington “recognized from the start” that its objectives of ensuring that Ukraine remained a viable sovereign state, maintaining NATO unity, and avoiding a U.S.-Russian war “were perhaps not the same objectives Ukraine had.” As a result, U.S. officials felt reluctant to push Ukraine toward diplomatic negotiations, especially while the Ukrainian military was advancing and congressional support for providing security assistance remained high.
Hard conversations with allies were also missing in the early days of the conflicts in the Gaza Strip and Yemen. The United States did not demand in the weeks after October 7 that Israel develop a “day after” plan for governance in Gaza once the Israeli military had sufficiently degraded Hamas—or even agree on what degrading Hamas meant. Had it done so, Washington could have had much more influence over how Israel prosecuted the war, insisting that non-Hamas security and political forces be allowed to start operating in Gaza once Israeli forces had eliminated large numbers of Hamas leaders in certain areas. Instead, as the war went on, the gap widened between the United States’ aim to prepare for post-Hamas governance in Gaza and Israel’s aim to destroy Hamas and prepare for military reoccupation.
Similarly, at the outset of the conflict in Yemen in 2015, Saudi Arabia and the United Arab Emirates promised the Obama administration that it would be over in six weeks. U.S. officials did not insist on seeing its partners’ plans for negotiating with the Houthis, particularly how they would handle the Houthis’ demand for inclusion in Yemen’s government. Washington spent most of the first few years of the conflict ramping up its support for the Saudi-led coalition rather than pushing for negotiations that could address the war’s political roots. One exception was a brief window at the end of the Obama administration, but at that point the parties simply stalled until the Trump administration took office. The Biden administration was also able to broker a cease-fire that lasted until October 7, 2023, but the war subsequently resumed. The Houthis are now stronger than they have ever been.
A NARROW WINDOW
U.S. public and political support for aiding a partner’s fight also diminishes over time. Some of this erosion is a result of general war fatigue (and fatigue with providing foreign assistance), but support can decline more quickly if Americans consider the partner’s conduct in war unjust. In Yemen, for instance, bipartisan congressional support for the Saudi-led coalition collapsed within a year of the conflict’s start. During that time, depending on the source of the tally, anywhere from 3,000 to 9,000 civilians were killed, and by most estimates, over 60 percent of these deaths—as well as significant destruction of civilian infrastructure such as roads and factories—could be directly attributed to air attacks the U.S. military was supporting. By 2016, senators from both parties were publicly questioning why the United States was supporting these airstrikes. In 2019, Congress passed a war powers resolution directing the president to prevent U.S. forces from providing any type of logistical support for the bombing campaign, including aerial refueling and intelligence sharing. (President Donald Trump vetoed it, and Congress did not override the veto.)
In Israel’s case, American political and public support started off extremely high but soon began to drop sharply. A Gallup poll in the fall of 2023 found that 50 percent of Americans approved of Israel’s military campaign and 45 percent disapproved; some polls found even higher levels of approval. Yet civilian casualties in Gaza and Israel’s destruction of infrastructure and restriction of humanitarian aid quickly raised concerns. The Biden administration was only able to change Israel’s conduct on limited occasions, and it focused most intensively on improving humanitarian conditions, not on trying to reduce the civilian harm of Israeli military operations. During the first weeks after the October 7 attacks, for instance, it persuaded Israel to reverse a decision to cut off Gaza’s water, electricity, and Internet access, and in April 2024, after Israeli forces killed humanitarian workers from the U.S. food relief organization World Central Kitchen, U.S. President Joe Biden angrily called Israeli Prime Minister Benjamin Netanyahu and secured a temporary lifting of Israel’s restrictions on humanitarian aid. It was not enough to halt the decline in American approval of Israel’s campaign in Gaza. According to Gallup polling, 55 percent of Americans disapproved of the military operations in the fall of 2024, and 60 percent disapproved a year later.
Support for Ukraine has followed a different pattern. The conduct of the Ukrainian military has never been seriously contested, and most Americans agree that Moscow is the aggressor and that Kyiv is fighting a just war with important geopolitical consequences. Even so, the protracted congressional debate over a supplemental funding bill from September 2023 through April 2024 showed how support can slip. After Trump, the Republican presidential candidate by March 2024, questioned the amount of aid being sent to Ukraine, many Republicans in Congress wavered, catching Biden administration officials off guard. But as one official put it, “Ukraine fatigue” had already set in across segments of the Republican Party—Trump had merely activated it.
In every conflict, U.S. policymakers have a short window of opportunity before domestic approval for supporting a foreign war begins to shrink. As public backing wanes, it becomes increasingly difficult to pass bills in Congress that provide an ally with security or economic aid. The president begins to face pressure from the electorate to display less public support for the ally, which harms Washington’s relationship with the partner it is trying to both assist and influence.
MINDS OF THEIR OWN
Another factor limiting the United States’ influence over the course of an ally’s war is the partner’s ability to withstand American pressure. Strong foreign leaders often use congressional relationships, public diplomacy, and media access to resist Washington’s demands. Netanyahu has a long history of employing such tactics and did so again during the Gaza war. In 2024, he launched a public relations campaign to criticize Biden for holding up weapons transfers to Israel—even though the only delay involved the shipment of 2,000-pound bombs that the Biden administration deemed too large to be used in a place as densely populated as Gaza. Ukrainian President Volodymyr Zelensky has displayed similar skill. Zelensky’s direct appeals to Congress for specific weapons pressured the White House to approve these transfers in the first years of the war, as did his criticism of the Biden administration’s delays in delivering more advanced systems. Worried about feeding Zelensky’s narrative that Washington was not bold enough in its support, some American officials hesitated to push Kyiv toward negotiations or to apply pressure when the U.S. military disagreed with Ukraine’s operational plans.
By acting too timidly around its allies, Washington has at times missed opportunities to change their behavior. When the bill for supplemental aid to Ukraine faced headwinds in late 2023 and 2024, Biden might have seized on the occasion to speak frankly with Zelensky about the limits of American support and the urgency of exploring negotiations while public backing in the United States remained strong. The Biden administration could have sent more technical experts and midlevel diplomatic staff members to Israel to convey concern about Israel’s rules of engagement as early as late 2023, when it was clear that the two countries were not on the same page about Israel’s choice of targets, choice of weapons, and approach to its first two major offensives, in Gaza City and Khan Younis. Later efforts of this kind appeared to work, as when the Pentagon persuaded the Israeli military to use limited strikes that led to less immediate civilian harm during the May 2024 offensive in Rafah or when the U.S. secretaries of state and defense sent a letter in October 2024 insisting that Israel lower its impediments to the delivery of humanitarian aid. Had Washington used similar tactics sooner, it might have had greater success at protecting civilians, ensuring that humanitarian access remained open, and pushing Israel toward operational strategies that would have limited civilian harm and been more effective against Hamas.
MISSED CHANCES
Part of the problem, however, is the structure of the U.S. government itself. Maintaining a coherent strategy across a sprawling national security bureaucracy becomes more difficult over the course of a long conflict. The president and his senior advisers often have an endgame in mind when Washington decides to support a partner’s war effort, but the task of building that partner’s military capacity and translating lofty political goals into a diplomatic strategy falls to bureaucrats much further down the chain of command. If these people are not given direct access to a partner country, they are not able to impart the United States’ objectives or learn more about the partner’s aims through the day-to-day practices of training and advising. U.S. allies can thus get their requests for military aid filled from afar, avoiding the forms of contact that would give the United States influence over their operational strategies and political goals.
During the war in Ukraine, for example, officials at the U.S. European Command, the National Security Council, the Defense Department, and the State Department have focused primarily on giving Ukrainians what they need to win on the battlefield. Meanwhile, without a large American diplomatic and military presence inside Ukraine, Pentagon and White House officials were sometimes left in the dark about Kyiv’s strategic and tactical plans. And the United States itself did not begin to seriously deliberate the terms on which the war might end—especially the scope of the bilateral security guarantees Washington might provide Ukraine—until 2024. In part, this was because there were serious disagreements within the government about whether the United States should push Ukraine toward negotiations at all.
By acting too timidly around its allies, Washington has missed opportunities.
Ultimately, it is up to policymakers to make use of the bureaucratic expertise and legal tools available to them to put pressure on foreign partners. During the war in Gaza, midlevel experts in humanitarian law and operational planners raised objections to Israel’s conduct, but they generally were not given access to the small circle of senior U.S. officials that made sensitive decisions about policy toward Israel. Key decision-makers did not make sufficient use of the extensive expertise across the Defense Department to guide the Israeli military on how make gains against Hamas while minimizing civilian harm in Gaza’s dense urban environments. And the Biden administration did not invoke the Leahy Laws, statutes that forbid the United States from helping foreign security units that violate human rights, or Section 620I of the Foreign Assistance Act, which bans security assistance to a partner blocking access to U.S. humanitarian aid, to pressure Israel. The administration therefore left on the table some of the tools that could have helped it compel its partner to behave differently in combat.
Washington’s failure to pull all the levers at its disposal may also have led to missed opportunities for peace. In 2024, for instance, the Biden administration delegated the task of getting Hamas to agree to a cease-fire and hostage release deal to its Arab allies rather than pressuring the group itself. It also largely avoided public criticism of the Israeli government when its partner rejected the same deal, in the belief that any American rebuke would harden Hamas’s position and make it more difficult to reach an agreement in the end. Netanyahu and Hamas’s leadership remained intransigent, and the war continued into 2025. The United States should never have felt the need to choose between pressuring its ally to end a war and pressuring its ally to improve its conduct during the war. Both are important and can be done at the same time.
The Trump administration did apply more pressure on Israel to agree to a cease-fire in October 2025, ending major combat. But its failure earlier that year to continue pressing Israel to allow access for humanitarian workers and supplies enabled near-famine conditions in parts of Gaza, and its more recent failure to press Israel for a workable postwar plan has allowed Israel to occupy up to 70 percent of Gaza without making any meaningful progress toward a stable political arrangement. Once again, not applying pressure meant the United States lost control.
In Ukraine, Washington may have waited too long to devise a strategy for ending the war. At first, many U.S. officials were so focused on ensuring that weapons deliveries to Kyiv would not trigger nuclear escalation that they lost sight of the need to put Ukraine in a strong negotiating position. Others were hesitant to press Ukraine to make a deal, uncertain that they could accurately judge the country’s military prospects after underestimating its ability to hold back a much larger, more powerful Russian army in the early weeks of the war. But some officials believe Kyiv could have negotiated a peace agreement in mid-2022 if Washington had pushed its ally harder to engage in negotiations after the success of its Kherson and Kharkiv counteroffensives. The conditions for decisive action were there: Congress still staunchly supported Ukraine, and Europe and the United States shared a strong commitment to the war effort and to NATO unity. Today, Ukraine’s recent military success may yet prove that trying to win on the battlefield, however long it takes, was the right strategy, but Washington is likely to have less influence over any negotiations than it would have had four years ago. The United States’ standing with its European allies has eroded substantially since Trump returned to office, and many of them will not want to see Washington, with its questionable support for NATO and European security, dictate the terms of peace.
GETTING BACK IN THE GAME
If Washington wants a say in how and when allies’ wars are resolved, it needs to find ways to work more productively with them while they are fighting. The most important lesson is both the simplest and the most difficult: American policymakers must use their leverage at the beginning of a conflict rather than wait for an opportunity that may never come. They should demand that a partner agree to a joint strategic plan as a condition of U.S. support, however difficult it may be to reach a consensus on war aims, postwar plans, and benchmarks for success. To formalize this joint plan, one U.S. official we interviewed proposed requesting that allies sign a written agreement reviewed every three to six months that lays out the terms of American support and the mutual obligations of both parties.
Another key to success is conditioning American security assistance on a partner’s adherence to the laws of armed conflict. When the United States knows that it will likely have concerns about a partner’s conduct—as when a war involves urban combat or the partner is fighting a nonstate group, such as Hamas or the Houthis, that itself abuses the laws of war—Washington should establish mechanisms as early as possible to adjudicate potential violations of the terms it sets. A standing review committee made up of lawyers and military experts should be insulated from political pressures and empowered to approve or deny security assistance based on clear benchmarks, including a partner’s meeting U.S. standards for military accountability and humanitarian access as established by the Leahy Law and the Foreign Assistance Act. These expectations should be conveyed to American partners at the start of the war.
U.S. policymakers must use their leverage at the beginning of a conflict.
The United States must also negotiate with the adversaries its partners are fighting. This is not a betrayal of American allies, but an essential part of making a settlement possible. U.S. partners are almost never able to end a conflict unilaterally. Washington’s tendency to focus on influencing its partner’s decisions while delegating engagement with the adversary to intermediaries—such as Qatar in the case of Hamas—ensures that U.S. leaders lack a clear picture of what it will take to successfully negotiate an end to war.
Finally, it is essential that the United States minimize conflict and confusion among its own government branches, departments, and agencies. When partners receive conflicting signals from different American officials, it is easier for them to play one U.S. agency off another. They learn to shop for the message they prefer and ignore ones they dislike. All levels of the U.S. government should be communicating and following one coherent strategy, from the president down.
For the foreseeable future, the United States is likely to remain heavily involved in its allies’ conflicts. Some partners may resist U.S. guidance on how and when to wage war, how to observe the laws of armed conflict, and which political outcomes best serve their security and Washington’s. Others, having witnessed the frictions that have emerged between Washington and its allies in recent years, are already following Israel’s and Ukraine’s examples in building up their own defense industrial bases to reduce their dependence on U.S. assistance. Yet in the end, when U.S. allies are faced with the prospect of war, many of them will still turn to the United States for the first-class military equipment and security, intelligence, and diplomatic support no other partner can offer. It will be up to Washington to ensure that when it decides to support a partner at war, it does so with a clear understanding of what leverage the United States has and how it can best use that leverage to keep its partner on a path that leads to the war’s end, on the best possible terms.
Loading…
