Slavery was the most important issue pushing the South to expand. Why was the Texas annexed? The reason was to expand slavery. Mexico wouldn't let the US have slavery before the Mexican war, which is why the US wanted Texas. David Wilmot claimed that Texas was the cause of the Civil War. He questions whether California and New Mexico will remain free. Senator Brown of Mississippi wants Cuba, and parts of Mexico for one soul reason, slavery. Charles Sumner said that the reason for Texas's quick annexation was for "Slave Power."

Secondly, foreign countries created a need for American expansion. Why was California needed so badly? America wanted to trade with the Orient. American trade greatly expanded. Oregon Territory was bought by the US to keep Britain and Russia out of America. The US didn't want even more competition from Britain and surely not Russia. The US wanted to buy Cuba to keep Spanish competition out of America. Nicaragua was needed for a proposed canal, to expand trade. But Britain also wanted Nicaragua. They came to an agreement saying that neither could build nor fortify around the proposed canal.

Westward expansion was also caused by a population growth. With thousands of immigrants pouring in, what was the US supposed to do? John Sullivan said that one hundred years America would have a population of "two hundred and fifth millions (if not more)." With a swell in population America had to expand.

America had the dire need to expand. Slavery, foreign countries, and population growth are the forces that drove America to expand in the 1840s and 50s. America expanded for these reasons.