The union started to control the mississippi river when troops captured vicksburg chancellorsville georgia gettysburg

Vicksburg is your answer

The city of Vicksburg was the last and key point in controlling the Mississippi river, and in effect, complete the Anaconda Plan, which split the Southern states into two portions to make them easier to be defeated.

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lincoln declared that slavery should not be allowed to spread into the new states, and of course this angered the south at the time.