On July 21, 1861, the Union and Confederacy fought at

On July 21, 1861, Union and Confederate armies clashed near Manassas Junction, Virginia

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.