Question

Why was there a need for expansion into the Pacific Ocean?

Answer 1

A--needing a place for steamships to refuel: the push came from the need for a modern navy to support the economic interests in Asia. 

Alfred Mahan's book on sea power convinced the US government a modern navy was needed. To create a large fleet, the US needed ports on islands throughout the Pacific to refuel and rest ships. These parts were provided as trade deals, protection, or takeover of the land.