When President Richard Nixon ordered U.S. ground troops to invade Cambodia on April 28, 1970, he waited two days to announce on national television the Cambodian incursion had begun. With resentment already building in the country over the conflict in Vietnam, the incursion felt like a final straw.

The news unleashed waves of criticism from many who felt the president had abused his powers by side-stepping Congress. By November 1973, the criticism had culminated in the passage of the War Powers Act. Passed over Nixon’s veto, it limited the scope of the Commander-in-Chief’s ability to declare war without congressional approval.

While the act was an unusual challenge, presidents since have exploited loopholes in the War Powers Resolution, raising questions about executive power, especially during states of emergency.

READ MORE: The US and Congress Have Long Clashed Over War Powers

Why Did the U.S. Invade Cambodia?

LISTEN: Nixon Orders Invasion of Cambodia

Cambodia was officially a neutral country in the Vietnam War, though North Vietnamese troops moved supplies and arms through the northern part of the country, which was part of the Ho Chi Minh trail that stretched from Vietnam to neighboring Laos and Cambodia.

In March 1969, Nixon began approving secret bombings of suspected communist base camps and supply zones in Cambodia as part of “Operation Menu.” The New York Times revealed the operation to the public on May 9, 1969, prompting international protest. Cambodia wasn’t the first neutral country to be targeted by the United States during the Vietnam War—the United States began secretly bombing Laos in 1964, and would eventually leave it the most heavily bombed country per capita in the world.

READ MORE: Why Laos Has Been Bombed More Than Any Other Country

The Cambodian Incursion (April-June, 1970)

Nixon approved the use of American ground forces in Cambodia to fight alongside South Vietnamese troops attacking communist bases there on April 28, 1972. Recent political developments within Cambodia worked in Nixon’s favor. Prince Norodom Sihanouk, who had led the country since its independence from France in 1954, was voted out of power by the Cambodian National Assembly on March 18, 1970. Pro-U.S. Prime Minister Lon Nol invoked emergency powers and replaced the prince as head of state in what became known as The Cambodian Coup of 1970.

On May 8, 1970, Nixon held a press conference to defend the invasion of Cambodia. He argued that it bought six to eight months of training time for South Vietnamese forces, thereby shortening the war for Americans and saving American lives. He promised to withdraw 150,000 American soldiers by the following spring. But Vietnamization was not going well, and the American public was fed up with the war in Vietnam. The invasion of Cambodia proved to be a tipping point.

READ MORE: Which Countries Were Involved in the Vietnam War?

Public Reaction to the U.S. Invasion of Cambodia

Daily News front page on May 1, 1970 showing demonstrators on campus of Ohio State University in Columbus standing their ground against national guardsmen. NY Daily News/Getty Images

Antiwar protests intensified across the country, particularly on college campuses. One hundred thousand people marched on Washington in protest. Approximately 400 schools had strikes while more than 200 closed completely. On May 4, 1970, the protests turned violent: National Guardsmen fired on anti-war demonstrators at Ohio’s Kent State University, killing four students and wounding nine. Ten days later, two students were killed at Jackson State University. The Kent State Shooting and the shooting at Jackson galvanized the country against the Cambodian incursion.

In Cambodia, the American bombing and invasion were weaponized as a recruiting tool by the Khmer Rouge, Cambodian Communist guerrillas who would later come to power in a brutal regime that would kill over two million people.

Congressional Reaction to the Invasion of Cambodia

Article 8, Section 1 of the U.S. Constitution grants the power to declare war to the legislative branch of the U.S. government—a purposeful departure from the British tradition of granting war-making powers to the king.

But the term “declare” has been open to interpretation for centuries. In practice, American presidents have been going to war without congressional approval for centuries. James Polk’s 1846 occupation of Texas helped kick off the Mexican-American War; Abraham Lincoln even authorized early military action in the Civil War without first seeking congressional approval.

The Cold War era saw new breaches in war-making protocol from the executive branch. “Congress had become increasingly active in the years prior to the passage of the War Powers Act,” says Fredrik Logevall, Laurence D. Belfer Professor of International Affairs at the John F. Kennedy School of Government at Harvard University. President Harry Truman did not seek Congressional approval before sending American troops to Korea, and when it came to the quickly-escalating Vietnam War, Congress was determined to play a larger role.

In late 1969, the Senate approved—by an historic vote of 78 to 11—the Cooper-Church Amendment named after Sen. John Sherman Cooper (R-Kentucky) and Sen. Frank Church (D-Idaho), prohibiting U.S. combat troops or advisers from operating in Laos or Thailand. “This was really the first time since U.S. involvement in Vietnam began that Congress had found the votes to limit the president’s ability to wage war in Southeast Asia,” Logevall says.

In June 1970, Congress repealed the Gulf of Tonkin Resolution in a vote of 81-10, reasserting their control over the president’s ability to make war. That December, Congress passed an amended version of the Cooper-Church Amendment. While neither action put an end to the bombing campaigns in Laos or Cambodia, they set a strong precedent for congress to reign in the president.

In June, 1971, Nixon received another blow to his war-making powers: The New York Times published the Pentagon Papers revealing that the U.S. government had secretly increased U.S. involvement in Vietnam.

READ MORE: The US and Congress Have Long Clashed Over War Powers

War Powers Resolution of 1973

Men of the U.S. First Cavalry Division in a jungle area 10 miles from the Cambodian border during a six-hour fight on March 30, 1970, shortly before President Nixon gave his approval for U.S. troops to cross over into Cambodia. Toshio Sakai/Bettmann Archive/Getty Images

The War Powers Resolution, also known as the War Powers Act, is a congressional resolution that limits the U.S. president’s ability to initiate or mount military actions abroad without the express approval of Congress. It passed in November of 1973 over Nixon’s veto and requires the president, as Commander-in-Chief, to notify Congress whenever armed forces are deployed and imposes a limit of 60 days on any engagements initiated without congressional approval. While it does not outright forbid presidents from taking military action, it does create some sense of accountability.

The War Powers Act allows the president to declare war under three circumstances: (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces. Since Nixon resigned less than a year after its passage in the wake of the Watergate scandal, it was up to future presidents to test its limits.

Did the War Powers Act Work?

“Since it was passed, the War Powers Act has been honored in the breach—that is, presidents have reported to Congress what they intend to do anyway and have mostly ignored the War Powers Act when it would have inconvenienced their plans,” says Andrew Preston, professor of American History at Cambridge University and co-author with Logevall of Nixon in the World: American Foreign Relations, 1969-1977.

“Indeed, presidents have almost dared Congress to do something about the lack of respect they've shown to the War Powers Act. If Congress's intention with the War Powers Resolution was to reduce American military intervention and to restore the balance between executive and congressional war powers, then it can only be seen as a failure,” Preston says.

Yet in 2008, a bipartisan movement to repeal the War Powers Act did not succeed. “In the power of the purse, Congress already has the power it needs to regulate presidential war plans,” says Logevall. “Congress has simply failed to use that power.”