The fact that the colonies were able to win the war was amazing in many ways. Before the war, the colonies had traded with each other, but they had no real political relations. When the war started, the United States had no real army and its only form of government was the Continental Congress. And yet this newly created government was able to defeat one of the world's Super power.
There are a number of reasons why the colonies won the war. The cost and difficulty of supply an amy across he Atlantic Ocean certainly caused some organizational problems for the British.