US Army no longer has any traditional infantry units. All are mechanized, not that they are always mounted. Not sure about Marines, but I imagine the same.
you're usually given everything you'll need to survive while on an operation.
I think standard units rarely carry for more than a few days worth of supplies. US military is built around out ?logisticing?/spending the enemy to a great extent. $400 dollar gallon of diesel and such. MIC and all.