The Nashville Convention is an American decision regarding the Civil War. The Nashville Convention was a meeting in Nashville Tenessee among 9 slave states following the annexation after the Mexican-American war, and it paved the way for the Compromise of 1850 (Clay and Douglas Draft). It can be taken before the civil war if slavery is allowed and average consciousness is above 6. It only shows up in the decision tab once consciousness hits 5. It reduces the consciousness of all states where 
slaves are allowed by 3.
Unlike real life, it is not a requirement, that the USA has annexed any Mexican territory, nor does it have anything to do with the American decision Clay and Douglas Draft.