Answer:
The reason why the Union troops eventually forced to retreat during the
Battle of Bull Run was because:
They were inexperienced warriors that didn't take their chances when they had the advantage and let the Confederates re-group, re-organize, and break the Union's line to defeat them
Step-by-step explanation:
First of all, the Union troops were the most disorganized. Second, they didn't strengthen their line when the final stage of the battle came up. Third, they let the confederates re-organize, re-group, and break their line on the west side. Therefore, they took advantage and forced the Union troops to retreat after having all the advantage to win the fight.