Tensions grew between the Union soldiers stationed there and the secessionist governor of the state, Claiborne Jackson. When the Civil War broke out, Jackson refused to send volunteers from the state to fight for Abraham Lincoln. Instead, the governor had the state militia muster outside the city to begin training in preparation to join the Confederate forces. The governor knew the most important resource in the state was the federal arsenal at St. Louis. Lyon realized what the governor was doing. On May 10, 1861, Lyon and his troops surrounded the pro-Confederate Missouri militia and forced them to surrender. While marching his captured prisoners through St. Louis, pro-Confederate citizens began to riot. When his German volunteers fired into the mob, 28 people were killed. The incident was called the Camp Jackson Affair. On May 17, Lyon was promoted to brigadier general. He was given command of the Union Army of the West. 