The 2500 men of the California Column traveled across the arid Southwest in staggered groups to allow water sources to replenish. In early 1862 Col. James H. Carleton sent units from Fort Yuma to Tucson, Arizona, which had recently been occupied by a Confederate force, Company A, Arizona Rangers. After a small engagement known as the Battle of Picacho Pass just north of Tucson between a detachment of Carleton's cavalry and Confederate pickets, the Union forces advanced on Tucson in three columns. They arrived in Tucson on May 20, forcing the heavily outnumbered Confederate garrison to withdraw to Texas without a fight.