On the afternoon of June 18, 1863, the city of Pittsburgh was thrown into a state of panic when rumors flew through the streets that Confederate General Robert E. Lee’s army had occupied the Shenandoah Valley in force — and was preparing an incursion into Western Pennsylvania.

In short order, Union General William T.H. Brooks, whose command of the Department of the Monongahela was located in East Liberty, rode into the city and confirmed to a swelling and “excited” crowd that he was had received an “urgent dispatch” from General Henry Halleck, the General-in-Chief of all Union forces at the War Department. “It is thought that an attack on Pittsburgh and Wheeling was imminent and it was recommended that both cities be put in a state of immediate defense,” Brooks told the crowd. …

As the forts in Pittsburgh were nearing completion, Lee’s 75,000 troops marched instead toward Harrisburg — with Philadelphia as the ultimate goal. But they were detoured unexpectedly when a few of divisions in search of shoes and other supplies encountered two brigades of Union cavalry just outside of Gettysburg.