Trump doesn't need congressional approval to launch operations shorter than 60 days, per the War Powers act, a law introduced by Democrats, by the way.
This isn't a simple operation, killing a foreign head of state is about as clear of a declaration of war as you can imagine. The law was introduced to put a check on the president's use of military force, it didn't give the president the power to declare war on another country.
https://en.wikipedia.org/wiki/War_Powers_Resolution