If you're referring to the militarization and aggression of Germany, there's a lot of debate as to why these nations didn't intervene earlier. The general consensus is that nobody believed Germany would take it as far as they did.
Answer:
The United States is a highly developed country, with a strong economy in pratically every single economic sector: from agriculture, to many types of industry, to high-tech, to services.
People from abroad demand U.S. technology and capital goods because they know that it tends to be of high quality. This benefits the American people because it raises U.S. exports, which brings more income to Americans in the form of either U.S dollars or foreign currency that is later converted into U.S. dollars.
This makes the U.S. economy more dynamic, not only for those directly involved in the exporting business, but also for many other people who benefit indirectly from such exchanges.
