Nathaniel Lyon attended the United States Military Academy and graduated 11th out of 52 in 1841. After graduation, Lyon participated in fighting the Seminole Indians in Florida, as well as in the Mexican American War, even though he did not support the conflict. During the war, he received several brevet promotions for gallantry under fire at the battles of Mexico City, Contreras, and Churubusco. He was then sent to posts in California where he participated in several Native American massacres. He was then reassigned to Fort Riley in Kansas, where he began to develop strong support for the Union as a result of the political climate developing in the state, known as “Bleeding Kansas.”
In February of 1861, Lyon was made commander of the Union arsenal in St. Louis, Missouri, where 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, Jackson had the militia muster outside the city to begin training in preparation to join Confederate forces. On May 10, 1861, Lyon and his troops surrounded the pro-Confederate Missouri militia under General D. M. Frost, and forced their surrender. While marching his captured prisoners through St. Louis, many citizens began to riot, and provoked the Camp Jackson Affair, during which Lyon ordered his troops to fire into the rioters. On May 17, 1861, Lyon was promoted to brigadier general and was given command of Union troops in Missouri.
Once in command of all Union troops in Missouri, Lyon began to pursue the capture of Governor Claiborne Jackson and the remaining Missouri Militia. On August 10, 1861 the Union forces met a combined force of the Missouri Militia and Confederate troops under the command of Ben McCulloch near Springfield, Missouri, during the battle of Wilson’s Creek. Nathaniel Lyon was killed during the battle while trying to rally his outnumbered soldiers. Although the Confederate forces would win the Battle of Wilson’s Creek, Lyon’s efforts prevented the State of Missouri from joining the Confederacy.