Final answer:
General Irvin McDowell was the Union officer whom President Lincoln forced into action, leading to the Union's defeat at the First Battle of Bull Run.
Step-by-step explanation:
President Abraham Lincoln forced General Irvin McDowell to take action, despite McDowell's warning that Union troops were not adequately prepared for battle. This action resulted in the First Battle of Bull Run, where McDowell's Union forces were defeated by the Confederate troops.
McDowell was under pressure to strike a quick blow to the Confederacy, specifically targeting the new capital of Richmond, Virginia, but the Union forces were repelled and the battle was a decisive Confederate victory.