It is up to Congress, not the president, to declare war; in 1973, Congress passed the War Powers Resolution to maintain its final say over the matter. Once a president initiates “hostilities” against a new enemy, the resolution gives him 60 days to gain congressional approval to continue his military initiative. If he fails to gain the approval of the House and Senate, he must stop his campaign within the next 30 days.

The president’s airstrike has set off the 60-day period.