Clark was also involved in the state militia, serving as a colonel in the Black Hawk War in 1832 and eventually rising to the rank of major general.

Clark was accused of conspiring to commit electoral fraud in the election and as a result almost fought a duel with Claiborne Fox Jackson, later a Governor of Missouri.

With the outbreak of the American Civil War in 1861, Clark, a wealthy owner of 160 slaves, became a leading secessionist in Missouri.

After leading his troops against Federal forces in the Battle of Carthage, Missouri on July 5, Clark was expelled from the House of Representatives for fighting against the United States.

Confederate Governor of Missouri Thomas Caute Reynolds did not appoint him to a second senate term due to allegations of alcoholism, mendacity, and womanizing.

[1] The author Kenneth E. Burchett notes that his legal knowledge was self-taught;[3] He was admitted to the bar in 1824 while residing at Fayette, Missouri.

The Democratic politician Claiborne Fox Jackson wrote a thinly pseudonymous letter accusing Clark of being complicit in the false ballots scheme.

Jackson transcribed a letter (including deliberately reproducing a spelling error) purportedly sent between Clark and James H. Birch.

[26] That year, he was elected to fill the vacant United States House of Representatives seat left by the resignation of James S. Green;[7] being sworn in on December 7.

[30] Jackson, who was now Missouri's governor, appointed Clark to command the 3rd Division of the MSG, which was located in the north-central part of the state.

Determining that the state capital of Jefferson City could not be held, they selected Boonville as the place to make a stand.

The Battle of Carthage, Missouri, was a minor affair in which Sigel's troops were driven from the field with little loss to either side.

[35] On July 13, Clark was expelled from his congressional seat,[2] for fighting against the United States, with an emphasis placed on his leading troops at Carthage.

[39] In early August, the combined Confederate and MSG commands moved to a camp along Wilson's Creek.

McCulloch demurred, but late on April 9, Price, backed by Clark and other MSG officers, pushed for an attack.

[41] Instead, Lyon attacked early on the morning on April 10, catching the Confederates and MSG by surprise and beginning the Battle of Wilson's Creek.

[43] Clark later tried to order the cavalry portion of his division to support his infantry, but the cavalry became greatly disorganized by Union artillery fire and the few who entered the fighting at this time instead fought with McBride's men or the South Kansas-Texas Mounted Volunteers, as they were able to locate them in the chaos of battle quicker than Clark's men.

[47] This government appointed Clark as a delegate to the Provisional Confederate Congress,[2] in which he served from December 7, 1861, to February 17, 1862.

The historians Ezra J. Warner and W. Buck Yearns describe the members of this Missouri delegation as "congress[men] without a constituency".

He sought to halt conscription in areas threatened by Federal forces, and supported the creation of partisan ranger units.

While he usually opposed providing Confederate president Jefferson Davis with appointive powers, Clark generally voted pro-administration on matters that did not affect his region.

[54] At the end of his term, Confederate Governor of Missouri Thomas Caute Reynolds[b] did not re-appoint Clark to the Senate.

[54] A number of allegations developed against Clark during his time in office, including alcoholism, disorderly behavior, mendacity, and in the words of Warner and Yearns, the "attempted seduction of Albert Pike's mistress".

The electorate was composed of Confederate soldiers and Missouri refugees, and Warner and Yearns refer to it as a "farcical poll";[49] Clark had the support of Reynolds's adversary Price in the election.

[48] After the Confederate defeat in 1865, a $10,000 reward was issued for Clark's arrest, but he evaded capture by using a disguise and a fake name to flee to Mexico.

[29] Having heard he was no longer wanted by the authorities,[29] Clark re-entered the United States and was in San Antonio, Texas, by September 1865.

[8] The requirement for the Ironclad Oath (a statement that one had been loyal to the US and had not participated in rebellion against it) to practice law was overturned by the Supreme Court of the United States in 1867,[61] but the Missouri Supreme Court issued a decision upholding a Missouri legal requirement to take the oath in order to hold political office.

[64] In 1872, he ran for a seat in the United States Congress one final time, but the nomination went to his son, who had been a general in the Confederate service.