The US colonists were able to defeat the strong British army for several reasons. One of the ways they were able to win was by getting assistance from other countries. The US received aid and military training from the French. The French were glad to help the US colonists, as they had a long standing rivalry with Great Britain.
Another reason why the US colonists had success was because of their determination to be independent. British soldiers, for the most part, did not care about losing the 13 colonies in America. The British government was more concerned with this factor. This is why the British government went as far as hiring soldiers from other countries to help them win the American Revolution. The British government paid roughly 30,000 German troops to help them maintain control of the colonies. The lack of passion for the war and the unfamiliarity with the landscape in the 13 colonies allowed the US to emerge victorious.