Vicksburg was essential to controlling the Mississippi River.
Explanation:
The city of Vicksburg in Mississippi was one of the most important locations that would help determine who succeeds in the Civil War. And it was the desire to conquer this city that Lincoln famously declared as "the key".
Vicksburg was along the Mississippi River and also a major stronghold of the Confederate troops. The capture of this city would determine which way the War will sway. Abraham Lincoln considered it important because it was essential to the controlling of the Mississippi River.
