It was very difficult to predict the outcome of the American Revolution. However, if anyone had tried, they would most likely had argued that Great Britain would win, as the country had a better army.
Several elements contributed to the victory of the colonies. One of these was the leadership of George Washington. Washington had previously fought in the French and Indian War, and although his performance was not always ideal, he was still able to gain skills of British tactics and develop his leadership and military skills.
Another element was the financial help from other countries. France set up a major program of aid to America, while the Spanish also secretly added funds.
Finally, the colonists' spirit and determination that came from the ideals of Enlightenment was vital. These people found their inspiration mostly in the French Revolution and the ideas of modern, liberal political philosophy.