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U.S. President Donald Trump arrives to announce military strikes on Syria as new National Security Advisor John Bolton fol...

When can a president use military force? The answer is complicated

If the Trump presidency is a series of tests of the balance of power, his use of military force in Syria may be among the most complicated yet. That’s because last week’s U.S.-led airstrikes against chemical production facilities in Syria were not authorized by Congress, were not in response to an imminent threat to U.S citizens, and were not given a green light by the United Nations, though the U.K. and France were involved in the operation as well. This raised a larger question: What are the real and theoretical limits on a president’s power to use military force?

Here’s a look at some guiding laws and documents.

Constitution: The Constitution offers conflicting guidelines on when a president can use military force.. In Article I, it states: “The Congress shall have Power … to declare War and … to raise and support Armies.” But in Article II, it states that “the President shall be Commander in Chief of the Army and Navy of the United States … when called into the actual Service of the United States.”

Declarations of war: The United States has only been involved in five officially declared wars. It has engaged in authorized conflict (see below), however, on numerous other occasions.

Authorizations of military force: When hostilities fall short of a formal declaration of war, Congress has authorized the use of military force to send troops into conflict. It’s a long practice dating back to 1798, when President John Adams and Congress tussled over how to combat French ships that were attacking American merchants (ultimately Congress did grant an authorization for force in that case). Since then, Congress has passed roughly a dozen AUMFs, according to the Congressional Research Service. The most recent authorizations came in 2001, to fight terrorism, and in 2002, to engage in Iraq.

The War Powers Act: As opposition to the Vietnam War grew, Congress passed a new law in 1973 to limit presidential power to launch the military into armed conflict. The War Powers Act became law after Congress overrode President Richard Nixon’s veto. It allows the president to use military force only with congressional approval or in response to a direct threat; requires the president to notify Congress within 48 hours of putting troops into hostilities; and limits such actions to 60 days without congressional approval, among other measures.

The problem with the War Powers Act? Multiple presidents have virtually ignored it or bent these rules, with no consequences. In 1986, Ronald Reagan attacked sites in Libya in retaliation for a bombing which injured Americans, but did not have congressional approval. In 1999, Bill Clinton launched airstrikes in Yugoslavia, aimed at preventing genocide, without giving proper notice to Congress or consulting lawmakers. In 2011, Barack Obama authorized military strikes in Libya, and argued that the War Powers Act simply didn’t apply.

Where are we now? As Sen. Chris Coons, D-Del., told NewsHour Monday, Trump’s moves in Syria represent “a grey area.” Courts have not laid down a clear line; the Supreme Court declined to hear a case brought by members of Congress against Clinton on this issue. In general, presidents have been able to take unilateral action without congressional approval. And Congress has not agreed on how or when to challenge that.

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