By keeping the governors from becoming so powerful that they could challenge Rome's authority.
The Treaty of Paris granted Guam, the Philipines, and Puerto Rico to the United States, with the US assuming the balance sheet of these territories.
The Teller Amendment was an amendment to the declaration of war on Spain which stated that the US would return Cuba to the Cubans following the conclusion of the war, but the US wanted indirect control over Cuba, and did not want its debt. It managed to negotiate that Spain would take on the 400 million dollars, and that the US would build a base on Cuba.
Scarcity of resources. The European countries and Japan didn't have all the resources they needed in order to keep up industrialization and a few other reasons. They resorted to taking over other lands in order the "cheaply" obtain the resources.
