the rapid postwar growth of the US economy required an increasingly centralized state to manage it. The more power that was concentrated in the executive branch and bureaucracy, the easier it was for the president to acquire territories abroad.
This culminated in the Spanish-American war in 1898, which ended with America acquiring a whole lot of different territories around the globe. America was officially a global power, one that intervened in a number of countries, made major diplomatic moves in East Asia, and played a critical role in ending World War I.
The next crucial step, though, came after World War II. The United States was the only country to emerge from the war in strong economic and military shape, and thus was in a unique position to shape the terms of the peace. The result was a global financial system, called the Bretton Woods system, aimed at coordinating the global economy and preventing another Great Depression — and the United Nations, created to preserve the postwar peace.