Nathaniel Lyon

Nathaniel Lyon (July 14, 1818 – August 10, 1861) was the first Union general to be killed in the American Civil War. He is noted for his actions in Missouri in 1861, at the beginning of the conflict, to forestall secret secessionist plans of the governor Claiborne Jackson.
He had fought in the Second Seminole War in Florida and the Mexican–American War. After being assigned to Kansas, where many residents were divided about slavery and the Union, he developed strong pro-Union views. In February 1861, Lyon was made commander of the Union arsenal in St. Louis, Missouri (another divided state). Suspicious of governor Claiborne, who was working with Jefferson Davis on a secret plan for secession, Lyon forced the surrender of the pro-Confederate militia.
Some civilians rioted and Lyon’s troops fired into the crowd, which came to be known as the Camp Jackson Affair. Lyon was promoted brigadier-general and given command of Union troops in Missouri. He was killed at the Battle of Wilson’s Creek, while trying to rally his outnumbered soldiers. But Lyon’s efforts had prevented the State of Missouri from joining the Confederacy.
https://en.wikipedia.org/wiki/Nathaniel_Lyon