War
You may have been watching a lot of movies, where the President declares war on a nation or a race of alien invaders. While watching that you may think it’s cool and dramatic, but in reality, it just isn’t the case. Alone, he can not just make this call for the rest of the country.
Congress again has to be the body of government that gives the go-ahead for the President to formally declare war. The leader is the one who signs the declaration of war and announces it, but he isn’t the deciding factor. Sorry space aliens, we just aren’t as cool as you thought we were.