Final answer:
The tension in the Missouri Crisis and after the Mexican American War was triggered by debates over the expansion of slavery.
Step-by-step explanation:
The tension that defined the Missouri Crisis and the territory gained after the Mexican American War was triggered by debates over the expansion of slavery. Both events involved disputes about whether slavery would be allowed in newly acquired territories, and these disagreements led to increased sectional tensions and the threat of disunion. In the case of the Missouri Crisis, the debate centered around the admission of Missouri as a slave state, which raised concerns about the balance of power between slave and free states in Congress. Similarly, after the Mexican American War, the acquisition of new territory raised questions about whether slavery would be allowed in places like California and New Mexico, leading to further divisions between the North and South.