Quoting from https://www.dialoguejournal.com/wp-content/uploads/sbi/artic...,
Contrary to what many have long assumed, Buchanan’s decision to intervene in Utah was not based on the despised Judge W. W. Drummond’s letter of resignation sent from New Orleans in early April 1857. That letter “recited nearly every accusation of Mormon disloyalty and perfidy that had accumulated during the prior ten years” and portrayed Utah as a “territory out of control,” with Governor Brigham Young as “the prime offender” (116). MacKinnon shows that the real catalyst for sending troops to Utah was “the substance and rhetoric in at least three other batches of material received in Washington during the third week of March 1857, weeks before the government was aware of Drummond’s resignation” (100).
The first decisive documents were the Utah legislature’s two memorials urging the appointment of only Mormons to territorial offices. As Utah delegate John M. Bernhisel reported to Young, they were seen as “a declaration of war,” breathing “a defiant spirit” and “not respectful” (106). The second document was a letter sent by Drummond probably before he left California for New Orleans and unrelated to his later letter of resignation. It detailed the impossibility of enforcing federal laws in Utah. Two more letters arrived the same week from Utah Judge John F. Kinney to Attorney General Jeremiah S. Black. Kinney gave more examples of the subversion of U.S. law in Utah and recommended that Buchanan replace Young as governor and establish an army garrison in the territory. The second Kinney letter enclosed a message from Utah’s Surveyor General David H. Burr, which included his dramatic assessment that any new governor risked assassination; Burr, too, recommended a military force.