Answer: D. Union troops panicked during a Confederate counterattack and retreated to Washington.
Explanation: On July 21, 1861, Union and Confederate armies clashed near Manassas Junction, Virginia. The engagement began when about 35,000 Union troops marched from the federal capital in Washington, D.C. to strike a Confederate force of 20,000 along a small river known as Bull Run. This battle cost some 3,000 Union casualties, compared with 1,750 for the Confederates. Its outcome sent northerners who had expected a quick, decisive victory reeling, and gave rejoicing southerners a false hope that they themselves could pull off a swift victory.