It would interfere with their trade alliances.
Communism leads to the government taking control of resources and industry. If it spread into areas the US did business, they could lose the ability to control resources and trade. This would eventually impact our political power and any military advantages the US had in Europe.
Banks loaned money to rich tycoons who were expanding the railroads
Answer:
The Civil War started because of uncompromising differences between the free and slave states over the power of the national government to prohibit slavery in the territories that had not yet become states. ... The event that triggered war came at Fort Sumter in Charleston Bay on April 12, 1861.