The United States Constitution is clear about which branch of government has the power to declare war. In Article I, Section 8, the Constitution states that “Congress shall have the power… To declare war.” But that simple statement has left room for interpretation, and centuries of American presidents have claimed the right to launch military attacks without congressional approval.

“The history of war powers has been a history of disputes between branches about what the meaning of ‘war’ is, what the meaning of Congress’s authority over war is, and what kinds of actions do and don’t count as war,” says Mariah Zeisberg, associate professor of law and politics at the University of Michigan, and author of War Powers: The Politics of Constitutional Authority.

When the Constitution was being written and debated, the framers clearly wanted to break from the British political tradition of investing all war powers in the executive (the king), but they also knew that legislatures could be dangerously slow to respond to immediate military threats. So instead of granting Congress the power to “make” war, as was first proposed, founders like James Madison changed the language to “declare” war.

Madison was no fan of executive overreach—“the Executive is the branch of power most interested in war and most prone to it,” he wrote to Thomas Jefferson—but that change of wording in Article I, Section 8 of the Constitution implied that the president, as commander in chief (Article II, Section 2), retained certain powers to “make” war, if not declare it himself.

In the early days of the United States, the understanding was that the president could order the military to defend the country against an attack, but that any sustained military action would require congressional approval.

It didn't take long before Congress and the president would clash over war powers. In 1846, President James Polk ordered the U.S. army to occupy territory in the newly annexed state of Texas. Congress recognized Polk’s move as a de facto declaration of war with Mexico, which claimed the territory as its own and vowed to defend it against an American “invasion.”

Congress ultimately granted Polk an official declaration of war, allowing for sustained military action. But the House of Representatives later censured the president for a conflict it believed was “unnecessarily and unconstitutionally begun by the President of the United States."

Even President Abraham Lincoln, a passionate defender of congressional war powers when he served in the House of Representatives, took liberties when taking his first military actions of the Civil War. While Congress was in recess in 1861, Lincoln issued proclamations to assemble Northern state militias and initiate a blockade of the South.

Lincoln admitted that he took these military actions without Congressional approval, later writing that “whether strictly legal or not, [the actions] were ventured upon under what appeared to be a popular demand and a public necessity, trusting then, as now, that Congress would readily ratify them.”

While Congress declared war six times (against six different countries) in World War II, President Harry Truman never asked for congressional authorization to send U.S. troops to Korea. Truman instead authorized the action under a United Nations resolution, claiming the conflict was akin to a “police action” not a “war.”

The war powers debate really came to a head during America’s involvement in Vietnam. In 1964, Congress authorized President Lyndon Johnson to use force in Southeast Asia in response to a North Vietnamese attack on American ships in the Gulf of Tonkin. The Gulf of Tonkin Resolution wasn’t a declaration of war, but that’s what was raging in Vietnam by 1973.

By that point, President Richard Nixon was in office, and the leaked Pentagon Papers revealed that Congress had been misled about America’s involvement in Southeast Asia. With public sentiment against the War in Vietnam, Congress passed the War Powers Resolution of 1973 to rein in presidential misuses of military power.

But if the War Powers Resolution was intended to, as it states, “fulfill the intent of the framers of the Constitution” and restore the war authority of Congress, it wasn’t terribly effective. The main provision of the law is that presidents can only take military action for 60 days before they need to get statutory approval from Congress, but it doesn’t stop presidents from acting unilaterally to put U.S. troops on the ground in the first place.

“After Nixon, it’s gone on from one president to the next—they believe they can use military force against one country after another,” says Louis Fisher, a visiting scholar at the William & Mary Law School who served for 35 years as senior specialist in separation of powers at the Congressional Research Service.

President Ronald Reagan invaded Grenada. President George H.W. Bush invaded Panama and Somalia. President Bill Clinton used military force in Iraq, Haiti, Bosnia, Afghanistan, Sudan and Kosovo all without congressional approval. (President George W. Bush didn’t declare war on Afghanistan or Iraq, but Congress authorized the use of military force for those engagements). President Barack Obama ordered targeted military strikes in Libya in 2011 and dozens of unmanned drone strikes in Pakistan without congressional approval.

While the War Powers Resolution has its limits, Zeisberg argues it is still legally and constitutionally significant.

“It’s been effective in the sense that professional executive branch lawyers in a functioning executive branch do internalize it and treat it as law,” says Zeisberg. “The War Powers Resolution has been useful in fostering a sense of transparency and accountability, and an idea of where the baseline is—what’s to be expected from a president.”