During the Gettysburg Campaign, Pennsylvania and New York activated state militia to defend the region against the incursion of the Army of Northern Virginia. It was not the only time emergency troops were mobilized in crisis conditions. In 1862 and 1863, four significant call-ups of troops for terms of three months or less responded to Confederate invasions, threats to border states, and the need to reinforce vulnerable Union positions. The data presented here are drawn from Provost Marshal General records and include only troops mustered into United States service—or, in Pennsylvania’s case, state troops recognized and paid by the federal government. Other state militia mobilizations that remained outside federal service were excluded from the official report and are not represented here.