Political compromise over slavery largely worked in the united states until:

Political compromise over slavery largely worked in the United States until Mexican territories were annexed to the US after the Mexican-American War, held between 1846 and 1848.

Explanation:

The Mexican-American War ended in 1848 with the Treaty of Guadalupe-Hidalgo, that established that Mexico would cede more than half of its territory, which comprises all of what are now the states of California, Nevada, Utah, New Mexico, Texas, parts of Arizona, Colorado, Wyoming, Kansas and Oklahoma. In addition, Mexico would relinquish all claims on Texas and the international border would be established on the Rio Grande.

The problem arose regarding these acquired territories and their condition regarding slavery, since there was a debate as to whether these territories should admit slavery or not. This controversy ended with the Compromise of 1850, by which California became a free state while Utah and New Mexico became slave territories.

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