While many have wrung their hands over Congress’s inability to check the Trump administration or provide adequate oversight of its handling of the covid-19 pandemic, there is one area where Congress has started to fight back after decades of letting the executive branch reign supreme. In recent months, we have seen revived congressional action seeking to limit the president’s ability to wage war without its input. In February, for example, the Senate introduced a resolution limiting the president’s authority “to attack Iran without congressional approval.”

If history is a guide, U.S. foreign policy works best when Congress exercises its constitutional oversight responsibilities and sets boundaries restraining unilateral presidential action in times of war or crisis.

Fifty years ago this week, President Richard M. Nixon authorized a military “incursion” into Cambodia, paradoxically expanding America’s war in Vietnam while attempting to withdraw from a long stalemated conflict.

While national security adviser Henry Kissinger had “no doubt about the operation’s success” — in reality, it proved of limited strategic value — the anniversary of the invasion offers valuable insights into the important role Congress has to play in U.S. foreign affairs. A role most current legislators have been avoiding for far too long.

Even before assuming office, Nixon determined that “total military victory was no longer possible” in Vietnam. The United States had been fighting in the Southeast Asian nation since 1965 and had suffered more than 48,000 military fatalities by the start of 1970. And yet the stalemated conflict seemed no closer to ending. Moreover, Nixon had concluded that to achieve his larger goals of reshaping Cold War foreign policy, the United States would have to free itself from the Vietnam quagmire before concentrating on improved relations with China and the Soviet Union.

One solution was to transfer wartime burdens to local leaders, a central feature of the “Nixon Doctrine.” As the president declared, the United States would “participate in the defense and development of allies and friends” but not “undertake all the defense of the free nations of the world.” Such proclamations implied, for perceptive observers at least, that there indeed might be limits to American power abroad.

Back in Vietnam, senior U.S. military commanders fretted about their already waning influence over the war’s direction. As more American troops departed the war-torn country because of Nixon’s “Vietnamization” policy, Gen. Creighton Abrams worried he was losing the battlefield initiative. Nor did the “secret” bombing of North Vietnamese sanctuaries inside Cambodia — the administration failed to inform Congress of its attack against a nominally neutral country — alleviate the communist threat to Saigon.

Itching to expand the war beyond South Vietnam’s borders, Abrams finally received the green light in late April for a cross-border invasion into Cambodia. American and South Vietnamese troops would cross the frontier, destroy enemy bases and supply caches and thereby provide the Saigon government additional time to prepare for the impending U.S. withdrawal. For the president, swift action by the allies not only would maintain pressure on the communists just across South Vietnam’s borders, but also clearly demonstrate his toughness to the enemy.

On April 30, 1970, Nixon, citing an “an unacceptable risk” to Americans not yet withdrawn from Vietnam, announced his decision to expand the war, although he claimed it was “not an invasion of Cambodia.” Without consulting Congress, the president argued he must be bold. “If, when the chips are down, the world’s most powerful nation … acts like a pitiful, helpless giant, the forces of totalitarianism and anarchy will threaten free nations and free institutions throughout the world.”

Nixon’s television appearance sparked a domestic firestorm that arguably marked the high point of the antiwar movement. College and university campuses were particularly hard hit. At Kent State, Ohio National Guard troops that had been called out to curb a student protest ended up shooting four of their fellow Americans. Similar violence at Jackson State University soon followed, and within weeks, more than 500 campuses were shut down by nervous administrators.

If Nixon was hoping his Cambodia decision would not be “assailed by counsels of doubt and defeat,” he surely had been disappointed.

Worse for the commander in chief and his military subordinates, the incursion realized few strategic results. Nixon might publicize the numbers of enemy weapons captured or tons of rice destroyed, but the operation proved more an inconvenience for Hanoi than a death blow. Unsurprisingly, the White House was mute on Cambodian civilian casualties.

More importantly — and why it still matters today — the Cambodian incursion spurred Congress into action. Emboldened by the domestic antiwar response, legislators reinserted themselves into the U.S. foreign policy decision-making process. Sens. John Sherman Cooper (R-Ky.) and Frank Church (D-Idaho) led the charge, introducing an amendment to the Foreign Military Sales Act of 1971 that would end U.S. support for allied forces operating outside South Vietnam’s borders. While ultimately failing in the House, the Senate’s approval of the amendment sent a clear message to the White House.

That summer, a bipartisan chorus called for repealing the 1964 Gulf of Tonkin resolution, which had given the president broad leeway in conducting military operations in Southeast Asia. Additionally, the McGovern-Hatfield resolution demanded an end to U.S. military operations in South Vietnam and the complete withdrawal of American troops. This vote also failed, yet the White House railed against congressional dissent. Nixon fumed that as commander in chief, he could act as the “sole organ of foreign affairs.”

While it is a stretch to argue this “congressional opposition” fatally undermined America’s war in Vietnam, the oversight probably helped limit further damage to Southeast Asia. Had it not been for the public and legislative reactions, an unconstrained White House would have been more inclined to use greater military force to cover the U.S. withdrawal.

Predictably, this more assertive role allowed Nixon supporters, both during and after the conflict, to “blame” Congress for a lost war in Vietnam. Surely, it was a lesson learned by many. Best to let the president have complete control over wartime decision-making so as not to be branded “unpatriotic” or unwilling to support the U.S. military.

Given the more recent hyperpartisan nature of American politics, it should come as no surprise then that Congress largely has been inattentive to its role in supporting America’s endless wars. But the Cambodian affair demonstrates that legislators can influence U.S. foreign policy if they put their mind to it.

Yes, congressional oversight of an imperial presidency comes with costs. There always will be some veterans and historians who believe, incorrectly, that Congress “lost” the war in Vietnam. But those accusations should not deter members from exercising their proper role, enshrined by the Constitution, by independently evaluating the potential costs and rewards of U.S. military interventions abroad.

American citizens deserve representatives who have the courage to fulfill their constitutional roles. And the erosion of constitutional authority and oversight should concern us all.

Fifty years ago, the president embarked upon a unilateral incursion into Cambodia, and Congress acted in opposition. The response from Congress was a clear reminder that foreign policy and warmaking are not the exclusive domain of the executive branch.