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.