It's all spelled out in a book called The Absent Superpower. The US is worried about the spread of Communism post-WWII, so we basically bribe other countries to take our side. Those who side with us are protected from invasion by the US military, have their shipping protected by the US navy, and are free to sell their products in the US. All that we ask in return is that they don't attack other members of our alliance and don't become Communist.