New York City during the American Civil War (1861–1865) was a bustling American city that provided a major source of troops, supplies, equipment and financing for the Union Army. Powerful New York politicians and newspaper editors helped shape public opinion toward the war effort and the policies of U.S. President Abraham Lincoln. The port of New York, a major entry point for immigrants, served as recruiting grounds for the Army. Irish-Americans and German-Americans participated in the war at a high rate.

Fort Lafayette was an island coastal fortification in the Narrows of New York Harbor, built offshore from Fort Hamilton at the southern tip of what is now Bay Ridge in the New York City borough of Brooklyn. The fort was built on a natural island known as Hendrick's Reef. Construction on the fort began during the War of 1812 and was completed in 1822. The fort, originally named Fort Diamond after its shape, was renamed in 1823 to celebrate the Marquis de La Fayette, a hero of the American Revolution who would soon commence a grand tour of the United States. During the American Civil War, the island fort became a prison, mostly for civilians viewed as disloyal to the Union; the fort became known as an "American Bastille." The fort was demolished in 1960 to make room for the Verrazzano-Narrows Bridge; the Brooklyn-side bridge tower now occupies the fort's former foundation site.