When the Civil War began in 1861, Philadelphia was initially divided over allegiances. The number of blacks living in the city was just 4 percent, or 22,000—small by modern standards but far greater than in any other northern city at the time. Philadelphia served as an important stop on the Underground Railroad, housing the largest free black population in the north. Large networks of African Americans and whites, especially among the fiercely abolitionist Quakers, assisted many slaves on the way to freedom. However, many Philadelphians were against the anti-slavery movement and abolitionists were sometimes the target of violence; some Quaker meeting houses were burned to the ground.

Ultimately, Philadelphia joined the Union and the city went on to play an important role in the war by supplying soldiers, ammunition, war ships, and army uniforms. More than 157,000 soldiers and sailors were treated within the city—many at Satterlee Hospital, the largest Army Hospital in the world at the time, which stood in West Philadelphia near the site of today’s Clark Park. The Mower General Hospital in Chestnut Hill also treated hundreds of thousands of injured soldiers.

Philadelphia began to prepare for invasion in 1863, but the southern army was held off at Gettysburg and ultimately the war was won. Philadelphia was less impacted by the Civil War than many other places, because it avoided the major physical destruction that many cities in the south suffered, as well as the major political and social upheavals that took place in other northern cities.