When the Mexican-American War began, largely through the actions of his old friend Polk, who was now President, Brown issued a call for 2,600 volunteers. When approximately 30,000 men answered this call, Tennessee's reputation as the Volunteer State was forever secured.
Like his friend Polk, Brown was also defeated for re-election as governor of Tennessee. He did not participate further in statewide elected politics, but was selected as a delegate to the Nashville Convention of 1850 held at Nashville's Maxwell House Hotel. This is probably the first place in Southern history where secession was ever openly and seriously discussed outside of South Carolina. Some historians credit the pressure that it instigated as helping to lead to the Compromise of 1850. He was a delegate to the Democratic National Convention in 1852 where Franklin Pierce and William R. King were nominated. Brown was subsequently rewarded for his service as a loyal Democrat by being appointed Postmaster General and was still holding this office at the time of his death. He is buried in Nashville's Mt. Olivet Cemetery. His speeches were published in Nashville (1854). [1]