Who decides when America goes to war?

The United States Senate's 50-48 vote to direct President Donald Trump to halt military action against Iran marked a rare effort by Congress to reassert its authority over questions of war and peace. Earlier in June, the House of Representatives approved a similar measure by a vote of 215-208.  Taken together, the House and Senate votes marked the first time both chambers of Congress have approved a war powers resolution concerning Iran, reflecting bipartisan concern over the extent of presidential authority to initiate or continue military action without explicit congressional approval.

The measure is unlikely to alter U.S. policy immediately. The White House argues that it lacks binding legal force, while supporters maintain that Congress has an independent constitutional responsibility to authorize military hostilities and determine whether U.S. forces should remain engaged in conflict.

The dispute centers on a question that has persisted since the founding of the republic. Who decides when the United States goes to war? Although the Constitution divides military authority between Congress and the president, decades of military interventions, counterterrorism operations and limited uses of force have blurred the line between legislative and executive power. The debate surrounding Iran has once again placed that unresolved question at the center of American politics.

The constitutional divide

The Constitution established a divided system of war powers intended to prevent any single branch of government from exercising unchecked authority over military action. Article I grants Congress the power to declare war, raise and support armies, provide and maintain a navy, and regulate the armed forces. Article II designates the president as commander in chief of those forces once they are raised.

Records from the Constitutional Convention indicate that the framers deliberately sought a middle ground between executive flexibility and legislative oversight. Delegates ultimately replaced language granting Congress the power to “make war” with the authority to “declare war,” preserving the president's ability to respond to sudden attacks while reserving broader decisions about initiating conflict for the legislature.

The United States has formally declared war only 11 times across five conflicts. Yet military engagements have become far more common than formal declarations, widening the gap between constitutional text and modern practice.

Rather than issuing declarations of war, Congress has often relied on Authorizations for Use of Military Force, or AUMFs, which provide presidents with legal authority to conduct specific military operations without formally declaring war. Since World War II, Congress has generally authorized major military operations through AUMFs rather than formal declarations of war, according to a May 2024 analysis by the Congressional Research Service. Such authorizations were used to support U.S. involvement in Vietnam, the 1991 Persian Gulf War, military operations following the Sept. 11 attacks, and the 2003 invasion of Iraq.

Congress responds to Vietnam

Congress attempted to address these ambiguities following the Vietnam War. Concerned that successive administrations had expanded U.S. involvement in Southeast Asia without a formal declaration of war, lawmakers passed the War Powers Resolution in 1973 over President Richard Nixon's veto.

The law requires presidents to notify Congress within 48 hours of introducing U.S. forces into hostilities and establishes a 60-day period after which military operations must cease unless Congress authorizes their continuation. Lawmakers viewed the measure as an effort to restore congressional participation in decisions involving armed conflict.

Its effectiveness, however, has remained contested. A 1999 Congressional Research Service report noted that the law originally allowed Congress to compel troop withdrawals through a concurrent resolution approved by both chambers but not the president. That provision was cast into doubt by the Supreme Court's 1983 decision in Immigration and Naturalization Service v. Chadha, which struck down the legislative veto as unconstitutional.

Although the Court did not rule directly on the War Powers Resolution, the decision raised questions about whether Congress could order military disengagement without presenting legislation to the president for signature or veto. Those questions continue to surround modern war powers disputes, including the current debate over Iran.

The gradual expansion of presidential power

The debate over Iran is the latest chapter in a decades-long expansion of presidential military authority.

A major turning point came in 1950 when President Harry Truman committed U.S. air, naval and ground forces to the Korean War following a United Nations Security Council resolution. Truman did so without seeking a congressional declaration of war, prompting criticism from lawmakers who argued that the decision bypassed Congress's constitutional role.

Subsequent administrations adopted similarly expansive interpretations of executive authority. U.S. involvement in Vietnam deepened through a combination of presidential decisions and congressional resolutions rather than a formal declaration of war. By the end of the conflict, concerns about the concentration of military authority in the executive branch had become a major factor behind passage of the War Powers Resolution.

Yet disputes continued. During the 1999 Kosovo air campaign, President Bill Clinton conducted military operations through NATO without specific congressional authorization. Members of Congress challenged the legality of the intervention, arguing that the operation violated the War Powers Resolution once the 60-day period had elapsed. Federal courts declined to resolve the dispute.

More than a decade later, President Barack Obama's administration advanced a similarly broad interpretation of executive authority during the 2011 intervention in Libya. Administration officials argued that the operation did not constitute “hostilities: under the War Powers Resolution because U.S. forces played a limited role and faced little risk of sustained combat. Many lawmakers disputed that interpretation, but the operation continued.

The same questions resurfaced during President Trump's 2017 and 2018 missile strikes against Syria in response to chemical weapons attacks. According to the Congressional Research Service, executive branch lawyers have repeatedly concluded that limited military operations may be conducted without congressional authorization when they serve important national interests and are not sufficiently extensive to constitute war in the constitutional sense.

Modern military operations have further complicated these distinctions. Counterterrorism campaigns in Yemen, drone strikes, special operations missions and the January 2020 strike that killed Iranian Gen. Qassem Soleimani all occupied a space between traditional peace and formally declared war. Rather than large-scale invasions authorized through congressional declarations, many contemporary military actions are limited in duration, geographically dispersed and justified through a combination of self-defense claims, existing authorizations and executive authority.

A debate without a final referee

Despite decades of disagreement, no definitive resolution has emerged. Presidents from both parties have maintained that they possess broad authority to use military force under certain circumstances, while Congress has periodically sought to reassert its constitutional role. The courts have generally declined to settle the dispute, leaving the boundary between Congress's power to declare war and the president's authority as commander in chief a matter of continuing debate.

The Senate's vote on Iran does not settle that debate. It does, however, place a fundamental constitutional question back at the center of American politics. More than two centuries after the Constitution divided war powers between Congress and the president, where one authority ends and the other begins remains a matter of dispute.