Albert Sydney Johnston was killed at The Battle of Shiloh, and was an important officer in the early part of the war. What command in the US Army did he hold before joining the South in rebellion?

Answer:   Johnston commanded the Department of The Pacific, headquartered in San Francisco.

In reaction to the secession of  South Carolina, Johnston put the garrison on Alcatraz Island on alert, and ordered the manning of the not yet completed fort at Fort Point, which today sits under the Golden Gate Bridge.

Johnston resigned to join the Confederacy on April 13, 1861, the day after the attack in Fort Sumter. His replacement was Edwin Sumner.