The 2001 Authorization for Use of Military Force has been used by four administrations to justify military operations in 22 countries.

9/11 reshaped US foreign and military policy for decades, shifting focus to non-state actors and the Middle East.

Presidents have often used military force without formal declarations of war

Although the Constitution grants Congress the power to declare war, American presidents have repeatedly ordered military action without formal declarations. These precedents have expanded executive authority in national security beyond what the framers likely envisioned.

A formal declaration of war automatically triggers hundreds of statutes granting the president sweeping domestic authority

Beyond authorizing military force abroad, a formal declaration activates a wide range of domestic powers, including authority over enemy property, trade restrictions, and emergency economic authority. It also waives the 60-day time limits under the War Powers Resolution.

The US and allies invaded Iraq based on flawed intelligence about potential weapons of mass destruction.

The president can deploy the military domestically with minimal oversight

The Insurrection Act of 1807 allows the president to send federal troops into American states during unrest or rebellion, even without a governor's consent. The modern use of the law grants broad discretion and limited congressional or judicial oversight.

The US launched the Global War on Terror after 9/11

In the wake of 9/11, the US declared a Global War on Terror—initiating military operations in Afghanistan and Iraq, expanding surveillance, and forming international coalitions to combat terrorism and disrupt extremist networks worldwide.

The War Powers Act may have ironically given presidents more discretion to engage in hostilities, not less

In this wide-ranging podcast on Congress's power to declare war, scholar Gary Schmitt argues that the War Powers Act, ironically, gave presidents a legal framework for initiating hostilities, creating a 60-day window for unilateral action that hadn't existed before.

NATO's Article 5 has been invoked once—after 9/11

The article was invoked the day after the attacks and launched a series of NATO campaigns to fight global terrorism. This page includes excerpts from a speech by then-NATO Secretary General Jens Stoltenberg about the attacks and the organization’s response.

The treaty's Article 5 requires mutual defense, but not necessarily military action.

How NATO responded to 9/11

The collective defense clause of the NATO constitution has only been invoked one time: after the 9/11 terrorist attacks against the United States. This video explores the immediate actions NATO took to respond to the attacks to support the United States. These firsthand clips from 2001 give a sense of the magnitude of the moment and the difficult decisions facing NATO leaders at the time.

NATO has only invoked its mutual defense clause once, in the days after the 9/11 attacks.

Congress has formally declared war 11 times across five conflicts, the last in 1942

The five declared wars were the War of 1812, the Mexican-American War, the Spanish-American War, World War I, and World War II. Every major American military conflict since Korea, Vietnam, Iraq, and Afghanistan has been fought without a formal declaration.

Explore every War Powers Resolution report submitted to Congress since 1973

This NYU School of Law project catalogs all unclassified 48-hour reports submitted by presidents since the War Powers Resolution was enacted, with search options by president, location, and claimed legal authority.

The first test of America's war powers came just a decade after the Constitution was ratified

During the undeclared Quasi-War with France in 1800, the Supreme Court ruled in Bas v. Tingy that Congress had authorized a valid "imperfect war" through statutes alone, without a formal declaration, establishing that there is a legal spectrum between peace and declared war.

Congress has unsuccessfully voted on war powers resolutions over Trump's military operations in Venezuela and Iran

After Trump ordered unilateral military operations in both Venezuela and Iran without congressional authorization, lawmakers from both parties introduced war powers resolutions to block further action. Each effort failed, with Republican majorities largely supporting the president.

Osama bin Laden declared jihad—a call for armed struggle framed as a religious duty—against the US in 1996.

The US never formally declared war in Vietnam

Instead of formally declaring war, Congress passed the Gulf of Tonkin Resolution in 1964, granting President Johnson broad authority to use military force. The resolution became the legal basis for escalating US involvement without an official declaration of war.

The War on Terror used controversial tactics like torture and secret prisons

As part of its post-9/11 strategy, the US employed enhanced interrogation, extraordinary rendition, and secret CIA black sites—practices widely criticized as torture and violations of international law and human rights.

The PATRIOT Act expanded government surveillance after 9/11

Signed into law weeks after the attacks, the PATRIOT Act granted federal agencies sweeping powers to monitor communications, access records, and conduct surveillance—sparking ongoing debates over privacy, civil liberties, and national security.

Presidents from both parties have resisted the authority of the War Powers Act

Presidents from both parties have treated the War Powers Resolution as voluntary, framing compliance as seeking "support" rather than "permission." Courts have declined to rule on the law's constitutionality each time a related case was brought before them.

A legal scholar argues that the existing war powers framework may collapse in the event of a major power conflict

The legal tests presidents have used to avoid seeking congressional authorization were designed for weaker adversaries. This piece argues that those tests fail against similarly armed competitors, where even limited strikes risk significant escalation, potentially triggering a constitutional crisis over who has the authority to respond.

War-making authority is divided between Congress and the president in the Constitution

Article I grants Congress the power to declare war and fund the military, while Article II makes the president commander in chief. However, the Constitution does not clearly delineate where one authority ends and the other begins, creating a tension that has never been fully resolved.

Truman's decision to enter the Korean War without congressional authorization marked a major shift in presidential war powers

Rather than seeking a declaration of war, Truman sent troops to Korea under his own authority, citing the UN Charter and his role as commander in chief. The decision set a precedent that subsequent presidents have relied on ever since.

The War Powers Act of 1973 was passed over Nixon's veto to limit the president's ability to wage war unilaterally

Passed in the aftermath of the Vietnam War, the War Powers Resolution requires the president to notify Congress within 48 hours of deploying forces and to end military operations within 60 days without congressional approval. Nixon vetoed the bill, arguing it was unconstitutional, but Congress overrode his veto.

The president has the sole authority to call for a nuclear strike

Under current US policy, the president alone can order the use of atomic weapons at any time without requiring prior approval from Congress—a power rooted in the president's role as commander in chief.

The power and influence of the American presidency have expanded significantly over time—far beyond what the founders envisioned.

The Iraq War, in photos

2023 marked the 20th anniversary of the start of the Iraq War. The U.S. carried out its first airstrikes in Iraq on March 20, 2003 – during a phase of ‘shock and awe’. Fires burning in and around Baghdad; U.S. soldiers on patrol; Iraqi...

The Barbary Wars were an early test of whether a president could wage war without a formal declaration from Congress

Jefferson sent naval forces against North African pirates without a congressional declaration of war, acting under his own authority. Years later, Madison took the opposite approach and formally sought congressional authorization.

Since World War II, formal declarations of war and peace treaties have become increasingly rare, replaced by cease-fires, authorizations of force, and "special military operations."

In his 1859 annual message to Congress, President Buchanan articulated a narrow view of presidential military authority

Buchanan argued that the president lacked constitutional authority to launch offensive military action without congressional approval, a view that reflected the prevailing interpretation of executive war powers throughout much of the nineteenth century.

The 1967 war was the first use of oil as a geopolitical weapon

Following the June 1967 Arab–Israeli war, Saudi Arabia, Kuwait, Iraq, Libya, and Algeria embargoed oil shipments to the US, UK, and West Germany. While the move did not have a large impact, it reshaped global oil flows, and foreshadowed OAPEC's later actions.

Explore the use of executive orders over time

An executive order is a presidential directive that manages the federal government and carries the force of law when authorized by the Constitution or statute. Since 1789, more than 16,000 have been issued—Franklin D. Roosevelt used the most, signing 3,721 during his 12 years in office.

Free speech has often been restricted in wartime, potentially unconstitutionally

From the Sedition Act of 1798 to the prosecution of anti-war activists during World War I, the US government repeatedly curtailed speech and dissent during wartime—actions widely criticized and, in some cases, later deemed unconstitutional.

President Bush addressed the nation on 9/11 from the Oval Office, watched by an estimated 80 million people.

9/11 permanently changed air travel security in the US with the creation of the Transportation Security Administration.

The attacks prompted a quick jump in American public support for war

In a Gallup poll taken days after the attack, an extraordinary 97% of Americans approved Congress declaring war on Japan—reflecting overwhelming national unity in the wake of the strike.

Nixon argued that the War Powers Resolution unconstitutionally restricted his authority

In his veto message, Nixon argued that the resolution would dangerously curtail the president's ability to respond to international crises and that only a constitutional amendment could properly define the limits of executive war powers. Congress disagreed and overrode the veto.

In the midst of the Civil War, Lincoln rapidly expanded presidential power

Abraham Lincoln suspended habeas corpus, expanded the army without congressional approval, and issued the Emancipation Proclamation—despite likely lacking authority to unilaterally alter national slave law. His actions set precedents for expanded presidential power in wartime.

The Supreme Court has never definitively settled federal war powers

Despite more than two centuries of conflict over war powers, the Supreme Court has consistently avoided defining the precise boundary between congressional and executive authority, leaving that boundary to be contested by the political branches.

The Prize Cases established that the president can respond to an attack without waiting for Congress to act

When Lincoln blockaded Confederate ports at the start of the Civil War without congressional authorization, the Supreme Court upheld his actions, ruling that a president does not initiate war by responding to an attack and need not wait for Congress when the nation is already under threat.

Members decide to what extent they respond to an attack on an ally

The NATO constitution is intentionally vague about what exactly is required from its members. Explore the historical debates and learn more about common misconceptions about NATO’s common defense clause.

The 1956 Suez crisis was one of the earliest tests of the rules-based order

When Britain, France, and Israel secretly invaded Egypt, the Eisenhower administration supported UN resolutions condemning the invasion and called for a ceasefire—publicly criticizing its closest allies to uphold the principle that force should not be used to resolve international disputes.

Madison was America's first president during a formally declared war

During James Madison's presidency, Congress formally declared war on Britain in 1812 over its practice of seizing American sailors, trade restrictions, and repeated violations of US sovereignty—marking the first time a sitting American president led the nation through a declared war.

In the 1930s, Congress nearly passed a constitutional amendment requiring a national vote before declaring war

The Ludlow Amendment, proposed amid rising isolationism after World War I, would have required a national referendum before Congress could declare war, except in cases of direct attack. It was defeated 209 to 188 in 1938 after Roosevelt lobbied against it.

US intelligence warned in multiple reports of al-Qaida's plans to strike before 9/11.

At the 1787 Constitutional Convention, Madison changed the draft language from Congress's power to "make" war to "declare" war

The change was deliberate: "make" war seemed to leave the president unable to defend the country against sudden attack, while "declare" preserved that defensive authority. The distinction between starting a war and responding to one has been debated ever since.

Nearly half of the discretionary funding in the US is allocated to national defense.

al-Qaida targeted multiple US interests in attacks before 9/11

Before 9/11, al-Qaeda carried out several deadly attacks on US targets—including the 1998 embassy bombings in Kenya and Tanzania and the 2000 bombing of the USS Cole—killing hundreds of people.

The president can federalize state National Guard units during national emergencies

While the governor usually commands their state's National Guard, the president can assume control—federalizing the guard—under laws like the Insurrection Act or Title 10. Once federalized, troops operate under the US military chain of command.

The Roosevelt Corollary transformed the Monroe Doctrine from a defensive warning into a rationale for US military intervention

In 1904, President Theodore Roosevelt declared that the US had the right to exercise international police power in Latin America. Over the following two decades, the US intervened militarily in the region about a dozen times—often enough that Marines became known locally as "State Department troops."