During the winter of 1777-1778, the Continental Army under George Washington camped at Valley Forge outside Philadelphia, where soldiers suffered from lack of food and clothing but were aided by donations from Americans. Foreign volunteers like the Marquis de Lafayette and Polish and Prussian officers helped train the army. The war later shifted south, where the British seized Savannah and Charleston, but American forces like Francis Marion used guerrilla tactics against the British. In 1781, the British under Cornwallis were trapped at Yorktown by a combined American and French force, leading to Cornwallis's surrender and the British conceding American independence in the 1783 Treaty of Paris.