The united states became involved in world war ii primarily because of events in

the Pacific

Explanation:

While events in Europe weighed heavy on the minds of American politicians, it was not until the bombing of Pearl Harbor in the Pacific Ocean in December of 1941 that war was declared by the United States in both the Pacific and European theatres of battle.

porter is writing economic history because he is focusing on the cost of developing vaccines.

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.