Final answer:
Lincoln was upset with McClellan at Yorktown because of his slow progress and overly cautious strategy, which allowed Confederate General Lee to regroup and prevented the Union from securing a decisive victory in battles such as Antietam.
Step-by-step explanation:
President Abraham Lincoln was upset with General George B. McClellan at Yorktown due to several reasons. McClellan's overly cautious approach significantly slowed the Union's momentum during the Peninsula Campaign. His initial plan was to land his army at Fort Monroe, Virginia, and advance towards Richmond by catching the Confederates by surprise from the east. This effort stalled when McClellan encountered Confederate defenses and worsened weather, ultimately halting after a battle against the Confederates. This delay allowed Confederate General Robert E. Lee to organize his defense around Richmond. In subsequent battles like Antietam, McClellan's reluctance to engage with his full force and inability to pursue and defeat Lee's army led to Lincoln's frustration, as the President desperately needed a decisive Union victory to turn the tide of public opinion and justify the Emancipation Proclamation.