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Under the United States Constitution, slavery was an issue left to the individual states. Each state had the right to decide for itself whether to sustain or abolish the institution. But whenever the United States acquired new land, the question of whether to sustain or abolish slavery was pushed onto the floors of the national Congress, because Congress itself administered territories. Congress generally did this by creating territorial governments (with a territorial governor appointed by the President) to maintain order, register land claims, and generally preside over the development of the territory. Territorial governments were regarded as temporary entities, which would eventually be replaced by full-fledged states. People in those new states would then be able to decide for themselves what they wanted to do about slavery. Everyone realized, however, that any decision at the time of statehood would be largely pre-determined by whether Congress in the interim had permitted slavery to exist under its territorial government. Clearly, if Congress banned slavery from a territory, there would be little likelihood that the eventual state would reinsert slavery into its initial constitution; and conversely, if Congress allowed slavery to take root during the territorial period, then the eventual state would be likely to make slavery legal in its initial constitution. Thus Congress had no choice but to address the issue of slavery in the territories, and all parties realized that the long-term stakes of Congressional decisions were high.

This module depicts the way Congress addressed slavery with regard to the two greatest land acquisitions in the nation's history: the Louisiana Purchase and the Mexican Cession. In both cases the issue of slavery deadlocked Congress, and in both cases the deadlocks were broken by political compromises. In the case of the Louisiana Purchase, the result was the so-called Missouri Compromise of 1820. In the case of the Mexican Cession, the result was the so-called Compromise of 1850 .