Answer
General Joseph E. Johnston was the initial target of McClellan's Peninsula campaign. McClellan's Union forces initially achieved success against Johnston's Confederate army, forcing them to retreat from Yorktown and Williamsburg. However, Johnston's skillful defensive tactics and McClellan's cautious approach led to a stalemate, and Johnston was eventually replaced by General Robert E. Lee, who launched a series of counterattacks that ultimately forced McClellan to withdraw from the peninsula.