The Congress had realized that without foreign aid Britishers would not be driven out of America. Therefore, a delegation was sent to France to negotiate an alliance for supplies and help. The French were also eager to help the Americans against their traditional enemy Britain. However, the French proceeded cautiously.
Louis XVI and his ministers waited till the news of victory of Americans at Saratoga reached them. This confirmed the belief of the French that Americans were capable of turning out the British. Consequently an alliance was concluded with the Americans on February 6, 1778. By virtue of this alliance the Americans agreed to protect the French holdings in West Indies in exchange for the assistance in trade, loans, military and naval aid. The treaty proved of immense value to the Americans in winning war against the British.
Prof. Handlin says, “The fruits of preparations were harvested in the three years between the autumn of 1778 and the surrender of Yorktown in 1781.”  With the help of the French the Americans won the War of Independence and expelled the British from the colonies and became their own masters. In 1778 the British Generals believed that the Americans could not withstand the British assault and would be soon crushed. But their hopes were belied and soon after the colonists were able to attain independence.
The final campaign of war took place in the South when General Clinton, who had succeeded General Howe, made a move. On hearing the news of French fleet approaching the Philadelphia, Clinton moved to New York and Washington followed him. The battle of Mammouth ensued which could not be won. Washington followed the British forces and won the battle at Stony Point (Connecticut) and saved the native population from the onslaughts of the British.
In the spring of 1789 British army captured Charlestown and General Cornwalis began to move further north in the hope of conquering each state one by one. In August 1780 Cornwalis defeated the American army under General Gates in the battle of Camden (North Carolona). Meanwhile the loyalists supporting the British and the patriots fighting for independence were involved in a Berocious Civil War in Carolina.
The two factions of American population engaged in guerella war, hitting each other. Ultimately patriots gained upper hand and defeated the loyalists army. Thus the patriots succeeded in establishing their control over Carolina. Meanwhile, Green was given command of the colonial forces in the South. He suffered a defeat at the hands of Cornwalis in the battle of Fuelford Court House in March 1781. The British too suffered heavy losses and retreated to Virginia. Thereafter Cornwalis took up position at Yorktown.
The time had come for the final contest between the two forces. The strategy was planned. The French troops landed in New England to launch an attack on Cornwalis. The French ships blocked the port of Yorktown and cut-off the supplies to Cornwalis by sea. The blockade diverted the attention of Clinton, who was in New York.
Meanwhile, the American troops under Washington and French troops under Lafayett advanced by land to town and surrounded the British. Cornwalis and his army was trapped. Co-nwalis seeing no hope of survival except surrender submitted on October 19, 1781. Thus the war ended in a decisive victory for the colonists.