Which compromise guaranteed that the slave trade could exist for 20 years?
The Missouri Compromise (1820)
The Missouri Compromise was a law created by Henry Clay that was intended to settle the dispute be pro and anti-slavery groups in Congress. The law drew a line at 36°30′ (the southern border of Missouri) and required that any new states added that were south of the line would be slave states, and any north of the line would be free states. Missouri would be the exception, and would be immediately added as a slave state.
The compromise was set in response to the growing gap between the free population in the North and their representation in Congress. The Constitution stated that slaves would count as 3/5 of a person in figuring out the population, which ended up giving the Southern States significant power in the Congress.
The Compromise had a strange result: Missouri was added as a slave state, Maine as a free state, and no other states were added until 1836 (Arkansas, slave). Mostly, it stopped the trend of emancipation moving south by drawing a legal line that protected the institution of slavery.
The law lasted until after 1850, when it was replaced by the Kansas-Nebraska Act, which kickstarted the Civil War.