Why US became the most powerful military and economic nation?

Why US became the most powerful military and economic nation?
After World War II the United States was the only country thatdidnt have all their factories obliterated. The world relied onthem for products and they also had the cash to lend for othercountries to rebuild. They have the biggest military as theycontinually build their military and have the biggest budget fortheir armed forces. Much of the world relies on them to keep thepeace so many nations intentionally do not build their militaryforces up.