Its just about the best videogame ever created. COD was the shit, UO just made it even better. For those that dont know, United Offensive is an addon for Call of Duty, a first person shooter based in WWII, that was released in 2003.
UO covers:
The Battle of the Bulge (this campaign is absolutely stunning in its flow and "suspense")
British Commando raids in Holland and Italy (with a mission where you ride as a gunner aboard a B17....very well done...get shot down, join resistance)
The Russian battle of kursk (Least favorite of the campaigns, but fun nonetheless)
New weapons: SVT40, TT30, Gew43, panzerschrek, flamethrower, MG34, .30 browning, .50 browning, brit revolver. Maybe more, cant remember, but the placeable machine guns are pretty sweet.
The multiplayer is what COD should have been in the first place. Single player campaign will have you playing non-stop for hours, although its fairly short. Good game, definitely worth it.