The final phase of the war, opening with the treaty of alliance with France on February 6, 1778, was confined mainly to the Middle states, the West, and the South. In the first sphere of action the chief events were the withdrawal of the British from Philadelphia, the battle of Monmouth, and the inclosure of the British in New York by deploying American forces from Morristown, New Jersey, up to West Point. In the West, George Rogers Clark, by his famous march into the Illinois country, secured Kaskaskia and Vincennes and laid a firm grip on the country between the Ohio and the Great Lakes. In the South, the second period opened with successes for the British. They captured Savannah, conquered Georgia, and restored the royal governor. In 1780 they seized Charleston, administered a crushing defeat to the American forces under Gates at Camden, and overran South Carolina, though meeting reverses at Cowpens and King's Mountain. Then came the closing scenes. Cornwallis began the last of his operations. He pursued General Greene far into North Carolina, clashed with him at Guilford Court House, retired to the coast, took charge of British forces engaged in plundering Virginia, and fortified Yorktown, where he was penned up by the French fleet from the sea and the combined French and American forces on land. =The Geographical Aspects of the War.=--For the British the theater of the war offered many problems. From first to last it extended from Massachusetts to Georgia, a distance of almost a thousand miles. It was nearly three thousand miles from the main base of supplies and, though the British navy kept the channel open, transports were constantly falling prey to daring privateers and fleet American war vessels. The sea, on the other hand, offered an easy means of transportation between points along the coast and gave ready access to the American centers of wealth and population. Of this the British made good use. Though early forced to give up Boston, they seized New York and kept it until the end of the war; they took Philadelphia and retained it until threatened by the approach of the French fleet; and they captured and held both Savannah and Charleston. Wars, however, are seldom won by the conquest of cities.