This is a US military surplus ALICE Canteen Pouch. This pouch was designed to hold the standard 1qt canteen, and features a drain hole in the bottom, ALICE clips on the back, and a small pouch on the front for storing purification tablets.
These pouches are US military surplus items in very good condition. They will only show minor signs of use, and will not have any problems that limit the intended use of the pouch.