Question

How did isolationism become quite impossible for the United States after the Spanish-American War?

Answer 1

The United States economy needed foreign markets to sustain itself, the federal government was under pressure to secure its power and prestige in the world community, and Americans yearned to be reassured of their national prowess. 

After the Spanish American War, the U.S. was intimately involved in Cuba and the Philippines as well as a player on the national stage.