Answer:
America wanted not just to contain communism - they also wanted to prevent the domino effect. Truman was worried that if Korea fell, the next country to fall would be Japan, which was very important for American trade. This was probably the most important reason for America's involvement in the war.
New technologies like steam engines, railroads, and telegraphs which made communication and transporting goods and materials across the country with ease turned many local businesses into national companies.
