The Continental Army was the national army of first the Thirteen Colonies, and then the independent United States, during the American Revolutionary War. The Continental Congress took a number of steps in the spring of 1775 to create the army in response to the Battles of Lexington and Concord in April and the seizure of Fort Ticonderoga in May. The units composing the Continental Army changed frequently, especially in the first two years of the war. From 1777 to the close of the war, the organization of the Continental Army became progressively more systematic and sophisticated. The Continental Army that served at Yorktown in 1781 bore very little resemblance to the Continental Army that blockaded Boston in 1775.