The U.S. Constitution assigns Congress the power to declare war, yet in practice it has allowed the presidency to steadily usurp that responsibility, raising urgent questions about how military action is authorized today. Over time, presidents have relied on expansive interpretations of their authority to justify military action without formal declarations by Congress.
“The last official declaration of war was in 1942, but the United States has gone to war many times since then,” said UVA history professor Andrew Preston.
The last official declaration of war was in 1942, but the United States has gone to war many times since then.
We recently spoke with Preston and Ashley Deeks, vice dean of UVA’s School of Law and a faculty senior fellow at the Miller Center, to examine the historical and legal implications of that shift and how it affects today’s decisions about war and peace.
Q: How have U.S. administrations typically engaged Congress when considering military action?
Andrew Preston: Since World War II, not very much. Starting with President Harry Truman’s decision to fight in Korea in 1950 without a declaration of war, Congress has often been an afterthought. When presidents do consider Congress, it’s usually in terms of avoiding obstruction rather than seeking genuine authorization.
Q: How does that square legally with the text of the Constitution?
Ashley Deeks: From the beginning, U.S. presidents were understood to have a “repel attacks” power separate from Congress’ authority to declare war. The framers anticipated emergencies in which the president, as commander in chief, would need to respond quickly without waiting for Congress. Over time, that power has expanded significantly as U.S. interests and perceived threats to U.S. security have grown.
The framers anticipated emergencies in which the president, as commander in chief, would need to respond quickly without waiting for Congress. Over time, that power has expanded significantly as U.S. interests and perceived threats to U.S. security have grown.
The Department of Justice’s Office of Legal Counsel has developed a two-part test for when the president can use force without Congress’ authorization. First, the president must identify a national interest. Second, the action must not rise to the level of “war in a constitutional sense,” which the executive assesses based on the anticipated nature, scope, and duration of the conflict. This framework gives the executive considerable flexibility and has become the operative rule in practice.
Q: Historically, has there been pushback?
Preston: When President Truman acted in Korea there was some, but not enough to stop him. Congress could theoretically block military action through its power of the purse, but that’s a drastic step. Once troops are deployed, few Congress members want to risk being blamed for denying them resources.
Q: War powers further evolved after 9/11 with the introduction of Authorizations for Use of Military Force (AUMFs). How did that change things?
Deeks: You can think of AUMFs in two ways. On one hand, they represent Congress’ interest in preserving a role for itself in authorizing force and imposing some constraints on that use of force. For example, the 2001 AUMF authorized force against those responsible for the 9/11 attacks or those who harbored them—initially, the Taliban and al-Qaeda.
On the other hand, the executive has interpreted the 2001 AUMF expansively over time, extending it to groups like ISIS and Al-Shabaab as “associated forces.” That allowed the president to claim congressional authorization for uses of force against those groups, even if they didn’t exist at the time of the AUMF.
This matters in part because of the War Powers Resolution, which requires the president to withdraw forces from hostilities after 60 days unless Congress authorizes the action. If the president interprets an AUMF as providing that authorization, the 60-day limit doesn’t apply.
Q: What factors influence when Congress forces an end to a war?
Preston: Congress tends to react when a war is going poorly. In Korea and Vietnam, opposition grew as those wars stalled or deteriorated. But it took years for Congress to act decisively.
Congress tends to react when a war is going poorly. In Korea and Vietnam, opposition grew as those wars stalled or deteriorated. But it took years for Congress to act decisively.
Q: On the congressional side, are there reforms Congress could enact to restore its role?
Deeks: The main issue is political will.
The Senate already has in place expedited procedures to authorize force or require troop withdrawal. That’s why Virginia Senator Tim Kaine has been able to repeatedly bring a War Powers Resolution to a vote pretty quickly.
For decades, scholars and think tanks have proposed reforms to the War Powers Resolution. Some might be achievable, such as setting stricter timelines and clearer obligations for the president to consult with Congress before introducing forces into hostilities.
Ambitious revisions would be hard to pass, though. For instance, more clearly defining “hostilities”—the trigger for the 60-day clock—would significantly constrain the executive, and there appears to be little political appetite in Congress right now to do that.
More clearly defining “hostilities”—the trigger for a 60-day clock—would significantly constrain the executive, and there appears to be little political appetite in Congress right now to do that.
Q: Over the past 75 years, why has so much power shifted to the presidency?
Preston: One reason is the nature of modern warfare, especially nuclear weapons. Decisions sometimes need to be made within minutes, which favors executive action over congressional deliberation. This has contributed to what scholars call the “imperialpresidency,” which means the president can act with significant autonomy, often using force first and seeking approval later.
Q: Does this suggest that the fast pace of the modern world is colliding with the slower, deliberative design of democracy?
Preston: That’s exactly right. The speed and complexity of modern threats are in tension with a constitutional system designed for careful, collective decision-making.