Like President Obama before him, Trump has claimed he can wage war without Congress' approval. Is that legally true? Here's everything you need to know:
Where is the U.S. fighting now?
The U.S. is currently conducting military operations in at least seven countries. In addition to Afghanistan and Iraq, American forces are involved in conflicts in Yemen, Somalia, Syria, Niger, and Libya. Most of these campaigns are being carried out under the 2001 Authorization for Use of Military Force (AUMF), which Congress passed just a week after the Sept. 11 terrorist attacks. The 60-word resolution granted the president sweeping powers to use force against any nation, organization, or person who aided in the attacks. That was understood to include Osama bin Laden's terrorist network al Qaida and the Taliban regime in Afghanistan. The Constitution gives Congress the sole power to "declare war," but since 2001, presidents have repeatedly used the AUMF to justify military operations around the globe. Secretary of State Mike Pompeo recently suggested President Trump had the authority to unilaterally order a war against Iran because of the AUMF.
Can Trump do that?
Legally, it's a big stretch. Pompeo has told Congress that Iran has "very real" ties with al Qaida. Intelligence analysts, however, are highly skeptical. Although Tehran allowed al Qaida operatives to remain within the country decades ago, there is no evidence of active collaboration between Iran's Shiite rulers and Sunni-dominated al Qaida. So, the Trump administration's trial balloon on Iran has alarmed members of both parties in Congress. "We don't want to be in forever wars in the absence of robust debate and approval by the Congress," said Rep. Matt Gaetz (R-Fla.), usually one of Trump's loudest champions.
How has the AUMF been used?
Presidents Bush and Obama invoked the AUMF at least 37 times for military actions in 14 countries and at sea. Obama invoked it to strike ISIS in the Middle East and al-Shabab in East Africa — terrorist groups that didn't exist on Sept. 11, 2001. Obama administration lawyers argued that those groups were "associated forces" of al Qaida, so the law still applied. But even before the AUMF, American presidents had steadily expanded their war powers over the last century.
Through what authority?
Through their role as commander in chief of the military. The Framers acknowledged that the president should be able to order a military response to an emergency, such as a foreign attack on the U.S., without a formal declaration of war. American presidents have steadily expanded the definition of "emergency"; as a result, the U.S. has had only five declared wars in its history, the last of which was World War II. President Truman didn't seek congressional approval for the Korean War — which lasted for three years and cost the lives of 40,000 Americans — by describing it as a "police action" under United Nations rules. The small American presence in South Vietnam metastasized into a full-blown war after Congress passed the 1964 Gulf of Tonkin resolution, authorizing the president "to promote the maintenance of international peace and security in southeast Asia." The undeclared war lasted until 1975, killing more than 58,000 Americans and more than 1.3 million people altogether.
Can Congress take its power back?
It has tried. In 1973, Congress passed the War Powers Resolution, which required the president to seek congressional approval for any conflicts lasting more than 60 days. Presidents have mostly just ignored it. Presidents Ronald Reagan, George H.W. Bush, and Bill Clinton all used loose interpretations of "national emergency" to justify interventions in conflicts in Lebanon, Grenada, Panama, Somalia, and Bosnia. Obama argued that the War Powers Resolution didn't apply to airstrikes in Libya because they wouldn't involve ground troops or the "sustained fighting or active exchanges of fire." The Obama and Trump administrations have used a similar rationale to justify supporting Saudi Arabia's brutal campaign against Houthi rebels in Yemen. Trump used the same legal framework for airstrikes on Bashar al-Assad's regime in Syria last year.
Why doesn't Congress object?
It often does, but it's nearly impossible to rein in the president without overwhelming, veto-proof congressional majorities. Congress passed a bipartisan bill in April demanding that Trump remove all troops involved in "hostilities" in Yemen, but the president simply vetoed it. The Democratic-controlled House passed a defense-spending bill last week that would end funding for Saudi forces in Yemen as well as force the president to seek congressional approval for striking Iran, but the same bill is unlikely to pass the Republican-controlled Senate. In many cases, Congress has actually preferred to let the president take the lead on military action abroad. When Obama asked Congress in 2013 to authorize military strikes against Syria in retaliation for chemical weapons attacks against civilians, the Senate refused even to hold a vote. "Taking a public stand on the use of force can be risky for members of Congress," says Elizabeth N. Saunders, a political science professor at Georgetown University. If a war goes bad, she says, the president is usually blamed. "Congress may like it that way."