See studies by A. E. Castel (1968) and R. E. Shalhoyse (1971).
The truce sought to forestall the outbreak of violence in the state of Missouri following the Camp Jackson Affair in which Union army troops under the command of Captain Nathaniel Lyon had captured Missouri State Militia training nearby. While marching the captured militia to the St. Louis Arsenal a civilian riot ensued. The soldiers fired into the crowd killing several. In the aftermath of the riot, the Missouri State Legislature enacted the governor's military bill replacing the Missouri State Militia with a new Missouri State Guard. The governor appointed Sterling Price commander of the Guard.
General Harney had been away from St. Louis during the Camp Jackson affair. Upon his return, he met with Price to sign a joint statement "for the purpose of removing misapprehensions and allaying public excitement," its object being "that of restoring peace and good order to the people of the State in subordination to the laws of the General and State Governments."
The truce drew the immediate ire of the politically connected Captain Lyon, who then successfully lobbied the War Department to relieve Harney of command on May 30. Whereas Harney was recognized as a moderate unionist, Lyon was an outspoken Radical Republican with ties to the paramilitary Wide Awakes organization. Lyon's elevation effectively ended the truce with Price.
On June 11th a last ditch meeting to prevent warfare was hosted at St. Louis by Missouri Governor Claiborne Fox Jackson. Jackson was an open Confederate sympathizer and Price had been a conditional unionist until the Camp Jackson Affair. During the meeting, Lyon angrily refused Price's requests to limit Union army exercises within the state in light of the massacre. Standing with his finger pointing at the Missouri government officials, Lyon shouted "I would see you, and you, and you, and you and every man, woman and child in the State, dead and buried. This means war. In an hour one of my officers will call for you and conduct you out of my lines." He then departed the negotiation.
As promised by Lyon, warfare soon began.