The dirty little secret is the USMCs use of Army Cav during the (second) Fallujah assault. They realized that the AAV was a joke for modern combat and used the Cavalry to lead off the assault with the M3 Bradley (better armor, better mobility, better gun, tow missiles). Over ONE THIRD of the troops in the (second) Fallujah assault were Army Cavalry troops.
The Army was in Iraq first and will be there last, as usual ;)
Actually, it wasn't much of a secret. The armored cav guys blew through the Jihadi defenses and occupied positions BEHIND them. This greatly reduced their ability to move around the city.
The Marines had the job of moving up block by block, kicking in every door and fighting at extreme close quarters. Army mech/cav battalions don't have enough rifleman to do that job...especially in a high casualty environment.