Complex sentence: Confederate pickets were waiting for the Union advance at Picacho Peak, their mission was to wait for the California Column and upon sighting, return to Tucson to alert Captain Sherod Hunter and his men.
Split into simpler sentences: Confederate pickets were waiting for the Union advance at Picacho Peak. Their mission was to wait for the California Column, and upon sighting it, return to Tucson to alert Captain Sherod Hunter and his men.