Final answer:
The Union army began actively recruiting African American soldiers to fight in response to the Emancipation Proclamation.
Step-by-step explanation:
The Union army response to the Emancipation Proclamation was that it began actively recruiting African American soldiers to fight.
The Emancipation Proclamation, issued by President Abraham Lincoln in 1863, freed enslaved people in Confederate territory. Following this, the Union army saw the opportunity to enlist African Americans as soldiers. By the end of the war, approximately 190,000 African American men had joined the Union army and played a crucial role in the Union's victory.
This response was a significant development as it marked the first time African Americans were allowed to serve as soldiers in the U.S. military.