American Civil War: Baltimore riot of 1861: A pro-Secession mob in Baltimore attacks United States Army troops marching through the city.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union. The central conflict leading to war was a dispute over whether slavery should be permitted to expand into the western territories, leading to more slave states, or be prohibited from doing so, which many believed would place slavery on a course of ultimate extinction.

The Baltimore riot of 1861 was a civil conflict on Friday, April 19, 1861, on Pratt Street, Baltimore, Maryland. It occurred between antiwar "Copperhead" Democrats and other Confederate sympathizers on one side, and on the other, members of Massachusetts and Pennsylvania state militia regiments en route to the national capital at Washington who had been called up for federal service. The fighting began at the President Street Station, spreading throughout President Street and subsequently to Howard Street, where it ended at the Camden Street Station. The riot produced the first deaths of Union volunteers by hostile action, although caused by civilians, in the American Civil War. Civilians among the attackers also were killed.

In the context of the United States, secession primarily refers to the voluntary withdrawal of one or more states from the Union that constitutes the United States; but may loosely refer to leaving a state or territory to form a separate territory or new state, or to the severing of an area from a city or county within a state. Advocates for secession are called disunionists by their contemporaries in various historical documents.

Baltimore is the most populous city in the U.S. state of Maryland. It is the 30th-most populous U.S. city with a population of 585,708 at the 2020 census and estimated at 568,271 in 2024, while the Baltimore metropolitan area at 2.86 million residents is the 22nd-largest metropolitan area in the nation. The city is also part of the Washington–Baltimore combined statistical area, which had a population of 9.97 million in 2020. Baltimore was designated as an independent city by the Constitution of Maryland in 1851. Though not located under the jurisdiction of any county in the state, it forms part of the Central Maryland region together with the surrounding county that shares its name.