What was the largest armed insurrection in the United States since the Civil War?

The largest armed insurrection in the United States since the Civil War is the Battle of Blair Mountain, which took place in 1921. This confrontation emerged from deep-seated tensions between miners seeking better working conditions and coal operators. Thousands of miners marched in armed rebellion against oppressive labor practices and a lack of union representation.

The significance of the Battle of Blair Mountain lies not only in its scale but also in its representation of labor unrest during the early 20th century. It involved roughly 10,000 coal miners armed with rifles and other weapons, and the confrontation was significant enough that federal troops were eventually called in to intervene.

The other events mentioned, while significant in their respective contexts of labor relations and civil unrest, did not reach the level of an armed insurrection comparable to the Battle of Blair Mountain. The Great Railroad Strike of 1877 and the Haymarket Affair were notable labor disputes, but they were not characterized by the extensive militancy and organization seen in the Battle of Blair Mountain. Similarly, the Battle of the Somme pertains to World War I and is unrelated to domestic labor struggles in the United States. Thus, the answer identifies the most significant instance of armed labor-related insurrection post-Civil War.