Combat operations are rarely as simple as Marines serving exclusively with Marines, or soldiers serving exclusively with soldiers. There’s no better recent example of this than Army Staff Sgt. Corey Calkins, who was awarded the Distinguished Service Cross for heroism serving alongside Marines. On Feb. 18, 2010, Calkins was serving in Marjah, Afghanistan, a Taliban stronghold that had been assaulted by Marine forces only days before. As part of a dismounted reconnaissance patrol consisting of U.S. soldiers, Marines and Afghan National Security Forces, Calkins led an attack on a platoon-sized group of insurgents in fortified positions in the bazaar near…