The Americans were able to prevail in the Revolutionary War (1775–1783) for three reasons.
First, British generals committed a number of serious mistakes. Many of them were overconfident. They also made tactical errors. For example, they sent their troops up Bunker Hill in an unimaginative and costly frontal assault. The American patriots inflicted heavy losses on the reckless British redcoats at Bunker Hill and fled only when they ran out of ammunition.
Second, the Americans had foreign help. American leaders knew that they were unlikely to win in the long run without foreign assistance. The capable and amiable Benjamin Franklin represented American interests in Paris. The French, eager for an opportunity to hurt the British, waited for the most propitious moment before intervening. In 1777, a British army of 6,000 had to to surrender at Saratoga. France formally entered the war in 1778. Spain and the Netherlands also joined the growing alliance against Britain.
Third, the Americans' military effort was led by the indefatigable George Washington. Washington made some errors on the battlefield early in the war, but he steadily improved as a commander. Moreover, he was steadfast: he kept the Continental Army together at Valley Forge and in spite of numerous defeats. Washington took advantage of a British mistake in 1781 and surrounded the enemy at Yorktown, which ended the war.