Before the civil war, the idea of popular sovereignty meant the territory’s status on free or slave state would be decided by the state judge majority vote by the people living there (state’s legislatures). the president the united states congress

 majority vote by the people living there (states' legislatures).

Explanation:

"Popular" means a vote by the populace, the people (or through their representatives in legislatures).

"Sovereignty" refers to where power or authority lies.

So "popular sovereignty" means that the people of the states had authority over their own territory's approach to the slavery issue.

The Civil War was fought over both the issue of "states' rights" (the idea of each state's popular sovereignty), as well as the issue of slavery, which was ended in the United States after the Civil War.   From that point on, at least on the issue of slavery, states could not decide their own path.  The 13th Amendment was added to the US Constitution and made slavery illegal in all states.

Here's what the 13th Amendment said:

north atlantic treaty organization.

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