Nathaniel Lyon (July 14, 1818 – August 10, 1861) was a United States Army officer who 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.

In 1850 he co-led the Bloody Island Massacre of 60–200 Pomo Native American old men, women, and children as part of the wider California genocide.

Suspicious of governor Jackson, who was working with Jefferson Davis on a secret plan for secession, Lyon forced the surrender of the pro-Confederate militia.

"[17] In March 1861, shortly before the outbreak of the American Civil War, Lyon arrived in St. Louis in command of Company B of the 2nd U.S. Infantry.

Lyon guessed correctly that Jackson would seize the federal arsenal in St. Louis if the state seceded and that the Union had insufficient defensive forces to prevent the seizure.

When the Civil War broke out and President Abraham Lincoln called for troops to put down the Confederacy, Missouri was asked to supply four regiments.

[17] Lyon himself had been extensively involved in the St. Louis Wide Awakes, a pro-union paramilitary organization that he intended to arm from the arsenal and muster into the ranks of the federal army.

Lyon was aware of a clandestine operation whereby the Confederacy had shipped captured artillery from the U.S. arsenal in Baton Rouge to the Missouri State Militia camp in St. Louis.

Lyon allegedly disguised himself as a farm woman to spy on the State Guard's camp and then claimed that he had uncovered a plan by Jackson to seize the arsenal for Missouri troops.

The event provoked the Camp Jackson Affair of May 10, 1861, in which Lyons' troops opened fire on a crowd of civilians injuring at least 75 and killing 28.

On June 12, 1861 Lyon (accompanied by Congressman Colonel Francis P. Blair Jr.) met with Governor Jackson and Major General Sterling Price of the Missouri State Guard (who both traveled under a safe conduct from Lyon) at St. Louis' Planter's House hotel to discuss the implementation and potential continuation of the Price–Harney Truce between Federal forces and the State Guard.

[19] The governor fled first to the capitol at Jefferson City (ordering the tracks destroyed behind him), and then retreated with the State Guard to Boonville.

[22][23] Lyon, badly outnumbered by Confederate forces, then dramatically led a countercharge of the 2nd Kansas Infantry on Bloody Hill, where he was shot in the heart at about 9:30 am.

[17] In the confused aftermath of the Union retreat from Wilson's Creek, Lyon's body was mistakenly left behind on the battlefield and discovered by Confederate forces.

[18] On December 24, 1861, the United States Congress passed a resolution of thanks for the "eminent and patriotic services of the late Brigadier General Nathaniel Lyon.

That the Thanks of Congress are hereby given to the brave officers who, under the command of the late general Lyon sustained the honor of the flag and achieved victory against overwhelming numbers at the battle of Springfield, Missouri.

"[18] The 24th Missouri Volunteer Infantry was recruited as "The Lyon Legion" in honor of the General, and carried a unique regimental color, depicting a Lion beneath a constellation of six stars.