“Can you believe this is only our day four here?” a corporal asks from his turret, more to himself than to the specialist in the turret next to him. They’re in an alleyway in the sprawling Baghdad suburb of Sadr City, straddling their up-armored Humvees, facing opposite directions, to cover the avenues of approach. They’re positioned in front of a three-story house where their platoon-mates regrouped in the aftermath of an ambush that killed one of their number — Sgt. Yihjyh “Eddie” Chen, 31, from Saipan — and left the unit cut off and surrounded.