McClellan's four corps commanders were Sumner, McDowell, Heintzelman, and Keyes, and they and he agreed upon a plan of campaign. The difficulties of transporting nearly 100,000 men to Fort Monroe were so great that two weeks were occupied in completing the transfer. In order to prevent the Confederates from getting in his rear, McClellan directed Banks to rebuild the railroad from Washington to Manassas and Strasburg, thus keeping open communication with the Shenandoah Valley, where the enemy were in force, a fact which caused the government much uneasiness for the national capital. Indeed, it was a part of the effective plan of Johnston to embarrass the campaign against Richmond. Banks occupied Winchester about the middle of March and sent a force under Shields to Strasburg. He found Stonewall Jackson there with such a strong force that he fell back to Winchester, where, after the withdrawal of the main body by Banks, he was attacked by Jackson, who was repulsed. In pursuance of the new plan of campaign, McClellan made Fort Monroe his first base of operations, using the route through Yorktown and West Point for the advance to Richmond. He expected to fight a great battle on the way thither, for the enemy could not fail to read the meaning of his movements. McClellan reasoned that this battle would take place between West Point and Richmond, and his intention was to advance without delay to the former position and use it as his chief depot for supplies. His plan was to make a combined naval and military attack on Yorktown, send a strong force up the York River, aided by the gunboats, and thus establish his new base of operations within twenty-five miles of the Confederate capital. It was not long before he began calling for reinforcements, and the government, instead of aiding him, took away piecemeal many of the troops upon which the commander had counted to aid him in his campaign. He wanted 150,000 men and a large increase of cannon. The 10,000 men, composing Blenker's division, were detached, as the President informed him, to support Fremont, but Mr. Lincoln promised to withdraw no more from the main army. McClellan remained at his headquarters near Alexandria until most of his forces were well on the road to the Yorktown peninsula. He left on the 1st of April and the troops were landed three days later. Then a force of 56,000 men with 100 guns started for Yorktown. But for the inherent timidity and distrust of McClellan, he might have captured Richmond, by marching straight ahead to the city, for the Confederate force opposed to him was but a fragment of his own, and could have been trampled underfoot. The Confederate intrenchments were a dozen miles in length, and were defended by Magruder with a force that allowed less than a thousand men for each mile.