Final answer:
General George B. McClellan led the western Virginia expedition in 1861, commanding the Army of the Potomac and later fighting in the Seven Days Battles.
Step-by-step explanation:
The Union General who led the expedition to western Virginia in 1861 was General George B. McClellan. McClellan was appointed as the general in chief of the Union army, holding overall control of the land forces and later taking direct command of the Army of the Potomac. Despite his cautious nature, which made him popular with his men, he faced criticism for his reluctance to engage in battle, fearing Confederate forces were too strong. In 1862, under pressure from President Lincoln and Secretary of War Edwin Stanton, a new effort was made to increase troop strength and McClellan was ordered to launch an invasion aimed at capturing Richmond. This campaign included the Seven Days Battles, where McClellan's forces faced General Robert E. Lee's Confederates in a bloody and brutal confrontation that ultimately resulted in McClellan's withdrawal from the Richmond area.