By the middle of the 1700s, the American colonists were very unhappy with the way they were being ruled by the English king.
For example, the British had imposed heavy taxes that the colonists believed were unfair. When the colonists refused to pay, the British sent troops to harass them into paying the taxes. The colonists began to resist.
Finally, on April 19, 1775, war broke out. This war, which lasted until 1781, was called the Revolutionary War (also known as the American Revolution). At the end of the war, Great Britain had lost, and no longer had control of the colonies.