Abraham Lincoln believed that the river fortress city of Vicksburg, Mississippi, was a key to winning the war. Vicksburg and Port Hudson were the last remaining strongholds that prevented full Union control of the Mississippi River. Situated on high bluffs overlooking a sharp bend in the river and called the "Gibraltar of the Mississippi", Vicksburg was nearly invulnerable to naval assault. Admiral David Farragut had found this directly in his failed operations of May 1862.