The second Trump administration has placed significant strain on U.S. treaty alliances. It has directly threatened the sovereignty of long-time allies such as Canada and Denmark and wielded tariffs as a weapon of first resort. Moreover, the administration has sought major force posture changes, often without allied consultation and reportedly based on ideological considerations rather than military requirements. And it has attempted to interfere in allied domestic politics, championing far-right parties and individuals.

To demonstrate that the administration’s turn against allies is not a settled fact of U.S. political life, the new Democratic majority in Congress should reassert powers as a co-equal branch of government. Rhetorical steps, such as reassuring statements and letters of concern, will not be enough even if they garner bipartisan support.

Instead, the next Congress must take practical steps to remove obstacles to defense industrial base integration and make it harder for the administration to take unilateral steps that sabotage alliances.

Current Senate Democratic leadership has telegraphed that working to improve the domestic economic situation in the United States is one of its top priorities for the new Congress. In this context, reasserting congressional authority over foreign policy in general and alliances in particular is not only a national security imperative, but also a part of the Democrats’ affordability agenda: President Donald Trump’s tariffs on Washington’s closest allies have added to Americans’ woes, while economic integration with allies can spur growth and jobs.

Amid rapidly increasing allied defense spending, everyone would benefit from integrating allies into the U.S. defense industrial base. Many of the Trump administration’s actions, however, have soured commercial relationships and pushed allies toward domestic or alternative suppliers. Congress can take concrete steps to help establish a more cooperative allied industrial relationship that ultimately improves intra-alliance burden-sharing.

First, Congress could treat more allies as part of the U.S. defense industrial base. This would involve adding more countries to the National Technology and Industrial Base and putting funding behind it; extending to other allies the exemptions to the International Traffic in Arms Regulations that Australia and the United Kingdom receive through AUKUS; designating additional key allies as domestic sources under Title III of the Defense Production Act (as Section 1080 of the 2024 National Defense Authorization Act did to add Australia and the U.K., alongside Canada); and raising the dollar thresholds for congressional notification on sales to close allies to match the increased price of modern high-tech weapons.

Before doing this, Congress would need to vet each ally’s technology protections and compliance with existing U.S. law on issues such as human rights—but for most treaty allies, this wouldn’t create a problem.

Second, Congress should facilitate allies building more together. It could increase funding for co-production of critical capabilities such as the Patriot PAC-3 and other air defense interceptors as well as counter-drone capabilities. To ensure that the Pentagon follows through, Congress could withhold a share of funds until the Defense Department certifies progress, the same mechanism that the Senate Armed Services Committee has proposed to ensure access to a second source for Patriot interceptor rocket motors.

The legislature could consider complementing bipartisan U.S. maritime security efforts, such as the SHIPS for America Act, with additional exemptions to 10 USC. 8679 for allies in Europe and the Indo-Pacific that can make major shipbuilding contributions.

Here, the Icebreaker Collaboration Effort Pact among the United States, Canada, and Finland offers a template: Finnish shipyards are building the first new U.S. Coast Guard icebreakers in decades as domestic U.S. production capacity ramps up. Any change must be coordinated closely with domestic interest groups, such as unions and industry, to protect American workers.

To pay for these steps, Congress could pass a supplemental appropriation aimed at building munitions production in the United States and allied countries. Such a supplemental could include a requirement that allies co-fund each co-production initiative they decide to join, which would go beyond existing proposals such as the nonbinding expectation in the fiscal 2026 National Defense Authorization Act (NDAA) that Baltic countries match U.S. funding for the Baltic Security Initiative.

Beyond specific measures related to the defense industrial base, Congress could examine authorities preventing deeper or more rapid cooperation, including limitations on how much allies can do with and for U.S. forces to support their readiness and operations. For example, Congress could extend foreign repair exemptions under 10 USC. 8680 to include South Korea and Japan, as the statute already permits under specified conditions for U.S. submarines in Australian and U.K. shipyards.

Other problems stem not from missing authorities but instead from open-endedness in existing authorities. For instance, Defense Secretary Pete Hegseth has reportedly not completed the required paperwork for allies to transfer their own Patriot interceptors to Ukraine. Any similar approval requirements that lack a deadline will give any administration an implicit veto over allied cooperation even if that cooperation advances U.S. interests.

Congress could consider reviewing these authorities systematically and develop more serious accountability mechanisms, such as instituting automatic approvals if the executive branch has not acted within a specified period and, at a minimum, establishing standing consent for transfers among treaty allies.

In other areas, Congress has already begun building the tools to create a bulwark against the administration’s efforts to undermine U.S. alliances. Section 1249 of the 2026 NDAA bars U.S. troop reductions in Europe below 76,000 without independent certification from both the commander of U.S. European Command and the secretary of defense that includes consultations with NATO allies. Congress should now overcome the administration’s objections to extend these provisions and establish similar limits on U.S. troop reductions in South Korea in the final version of the 2027 NDAA.

After the Supreme Court confirmed in February that presidents can impose tariffs only when Congress has clearly authorized them, the administration shifted to a temporary surcharge and then to Section 301 tariffs that include allies. Congress could consider passing into law a bill similar to the bipartisan Trade Review Act, which would make any new tariff lapse after 60 days without congressional approval.

Members have also proposed various ways to limit potential military action against allies, including a bipartisan bill that would prohibit the departments of Defense and State from using funds for military action or annexation of the sovereign territory of a NATO member state. Although the worst of the Greenland crisis, which inspired these legislative proposals, has passed for now, the administration could attempt to once again coerce allies at any point. Moving forward, Congress should learn from the fact that its efforts to rein in Trump through stand-alone bills and appropriations amendments have stalled. Instead, lawmakers should consider adding new protections to must-pass legislation.

Legislation is not the only tool, however. The Senate’s advice-and-consent powers allow it to withhold approvals on confirmations, and both chambers can conduct oversight hearings and expand investigations. For example, the Senate can resist confirming nominees selected for posts in Europe, up to and including all pending ambassadorial nominees or potential new cabinet nominees, until the administration accounts for its attempted interference in allied domestic politics. Next term’s committee leaders could also develop a joint hearing agenda early in the term that prioritizes alliance issues, including interference in allied politics.

Enacting fixes, however, is just the first half of the challenge. Ensuring that the administration complies is the other, perhaps harder, half—as the past 22 months have illustrated.

In October, a bipartisan group of senators wrote a letter to the secretaries of defense and state, urging the “swift obligation” of Baltic Security Initiative funding, following up on their similar letter sent in May that appears to have gone unheeded. In September, the U.S. Government Accountability Office found that the administration could not lawfully let congressionally appropriated funds expire unspent.

Congress can make its legislative priorities bite by withholding funds until the administration certifies progress, as several of the provisions discussed above do, setting firm obligation deadlines and authorizing multiyear procurement that is harder for any one president to unwind.

But while compliance mechanisms might be able to force the administration’s hand, they cannot force allies to continue investing in a partner that they no longer trust. Whether allies ultimately believe that they still have a partner in the American people depends on how muscular the next Congress decides to be.