The existence of United States of America as a separate independent country was declared during the American Revolution of 1789. Thus, Americans still had to fight for their independence in the War of 1812 against Great Britain. This war was considered to be America's second Revolution. And due to the expansionist issues over slavery after the Mexican-American War the conflict broke into a Civil War that devastated the Southern states.
Many consider the Civil War to be the Third American Revolution, which is a reasonable point due to the settlement of the argument of abolition of slavery during this war. The main reason that started the Civil War was the secession of Southern states from the Union in 1860-1861, after the election of Abraham Lincoln as president. Thus the battle fronts organized on both eastern and western sides of Appalachian Mountains and along the Atlantic coast of the states. Northern forces were much superior in numbers, equipment and supplies that were provided by the industrialization of the region and extensive railroad system. South had the only advantages of fighting on the home grounds and fighting a defensive war. In the course of battles Union naval fleet held a blockade of the Confederate shores, thus jeopardizing even the prosperous Southern cotton trade. On land, eastern front locked into a stalemate towards the end of 1862 between Washington, DC and Richmond, VA.
While, quite successful Unionist forces in the west occupied most of the Mississippi river. At that point Lincoln issued the Emancipation Proclamation that reshaped the war into the fight against slavery for the Union troops. Thus also allowing the recruitment of blacks into the army, Northern army experienced a good reinforcement that allowed it to push for victory in the war. Civil War proved to be the bloodiest war that US experienced so far, but the end was reached by May of 1865. North won and thus declared all slaves on the US terr.