The American colonies revolted against Great Britain because the Colonists claimed that they had been denied their rights as Englishmen. England had attempted to tax the colonies to pay the cost of the recently concluded Seven Years War (French and Indian War.) Americans resented the effort because they believed that they, as was the case by other citizens of Britain, should only be taxed by their duly elected representatives--in this case the colonial assemblies. A heated debate developed which culminated with the Battles of Lexington and Concord.
France, the traditional enemy of Great Britain, came into the war following General Gates' victory at Saratoga. France's participation soon compelled Spain and the Netherlands to come into the war also. It thus became a European as well as American War. Since British colonies as well as colonies of other nations, were scattered around the globe, the conflict became a truly global war.