The Washington PostDemocracy Dies in Darkness

Donald Trump declares ‘war’ on ISIS. But Congress probably won’t.

Donald Trump, presumptive 2016 Republican presidential nominee, pauses while speaking during a campaign event on veterans reform in Virginia Beach, Virginia, U.S., on Monday, July 11, 2016. (Andrew Harrer/Bloomberg)

This post has been updated.

In the hours after the Thursday night attack on Nice, France, Donald Trump called for a major shift in U.S. foreign policy.

Asked by Fox News's Bill O'Reilly whether he would move to declare war on the Islamic State, a.k.a. ISIS, Trump answered in the affirmative.

"I would," Trump said. "This is war. If you look at it, this is war."

Whether Trump meant to or not, he proposed something exceedingly rare in American politics. Congress hasn't declared war in 74 years, in fact. And it has only done so in five wars total -- the War of 1812, the Mexican-American War, the Spanish-American War, World War I and World War II.

The United States has engaged in many wars in recent decades, but it hasn't actually declared war since World War II. The last time the U.S. officially declared war was on six countries, according to the Senate historian, in 1941 and 1942.

Congress has authorized the use of military force -- which isn't a declaration of war but has many of the same practical implications -- against al-Qaeda, which was used in Afghanistan. The al-Qaeda authorization is being used to fight the Islamic State in Iraq and Syria, but some in Congress and President Obama want a new authorization of military force (AUMF) for the fight against ISIS.

Congress has resisted the idea of even voting on a new AUMF -- much less a declaration of war -- in part because many lawmakers paid a political price for supporting the Iraq war. Among them was soon-to-be-Democratic presidential nominee Hillary Clinton, whose 2008 Democratic primary campaign was hamstrung by her vote in favor of the use of force in Iraq. She eventually apologized for it.

Those pushing for a new AUMF, besides Obama, include potential Clinton vice presidential pick Sen. Tim Kaine (D-Va.), who is sponsoring an AUMF bill in the Senate.

So what's the difference between an AUMF and a declaration of war? The Atlantic explained in 2013:

A "declaration of war" has always been a specific policy tool -- a blunt one, and one that many presidents, and Congresses, have chosen not to use. "Authorizations," by contrast, permit the two branches to agree on limited war aims. An authorization can lapse without a formal surrender; it can permit military action short of total war. It's a tool that any government needs, and any rational constitution provides.

The United States already considers itself in an international “armed conflict,” with war-making powers, against the Islamic State in Iraq and Syria, where U.S. air attacks and some ground operations have been underway since September 2014. Other air attacks have been carried out against allied groups in Yemen, Somalia and Libya, as well as by U.S. forces in Afghanistan.

The administration says legal justification for those operations falls under the 2001 AUMF against al-Qaeda, as well as the U.S. right to self-defense and the president’s constitutional powers to protect the country. In February 2015, Obama sent lawmakers a proposed new authorization specifically against the Islamic State.

It has been 25,640 days since an American soldier died in a war declared by Congress

Congress hasn't acted on the measure and has failed to agree on an alternative. Democrats said Obama’s measure was too expansive and might be used by a future administration to justify overseas adventurism. Republicans said it limited presidential powers against a threat that has spread across the world.

It remains to be seen whether Trump actually wants to push for a step beyond an AUMF and really advocate for a declaration of war.

Then again, Trump is a guy who does like to take things further than other politicians.

Karen DeYoung contributed to this post.