With last year's Call of Duty, Activision and developer Infinity Ward proved that there's room for more explosive World War II-themed action franchises on the PC than just Medal of Honor. Now an expansion pack for Call of Duty, titled United Offensive, is in development at Gray Matter, itself a veteran of WWII games with the critically acclaimed Return to Castle Wolfenstein. Activision gave us an updated look at United Offensive recently to show off some of the impressive additions Gray Matter has made to Call of Duty's already solid core gameplay.

　　Gray Matter is doing impressive things with the Quake III engine in Call of Duty: United Offensive.

　　United Offensive may be an expansion pack in name, but it'll bring a lot more than just new maps to the table. In addition to the expected array of new rifles and sidearms, a number of more unique weapons will join the mix, such as a flamethrower, a bazooka, a portable machine gun, and a smoke grenade. To use the machine gun, you will need to go prone or mount it on a windowsill to fire it.

　　The grenade will come in especially handy in combination with the new limited sprint ability, which lets you run forward at great speed at the cost of a lowered weapon. Cooking a grenade will let you control how long you hold it after you've pulled the pin, so you can ensure no wily enemies scoop it up and toss it back before it explodes.

　　Like the single-player campaign in the previous game, United Offensive's single-player campaign is split into thirds between British, American, and Russian forces, and these parts will focus on the invasion of Sicily, the Battle of the Bulge, and the Battle of Kursk, respectively. We took a look at two previously unseen missions, one each from the American and British campaigns. In the American mission, the 101st Airborne attempted to move into the village of Noville against heavy German resistance, aided by an AI-controlled tank that did considerable damage to enemy forces before being taken out. This mission primarily involved house-to-house fighting in tight quarters and showed off several scripted action sequences. In one part, the player and some of his allies were sealed in a cellar and had to blast their way out with explosives to rejoin the combat.

　　Trace the Battle of the Bulge and the invasion of Sicily in United Offensive's single-player campaign.

　　In the British campaign, you'll be working with SOE and SAS forces covertly to accomplish a series of behind-the-scenes goals. We saw a mission set at dusk in which the player and an ally had to escape on motorcycle from pursuing enemy forces after perpetrating an act of sabotage. In this sequence, the motorcycle was driven by the AI while the player sat in a sidecar, blasting away at the oncoming assailants.

　　After cutting a swath outside of and then through the town itself, the player and his ally made their way to the docks and managed to steal a small boat to make their escape. The action wasn't over at this point, though, as the enemy also took to the water and the player had to destroy several pursuing boats before escaping to freedom. These two sequences were essentially rail shooters that were somewhat similar to the B-17 bombing mission we saw at E3 earlier this year.

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　　United Offensive will feature several rail shooter sequences, such as this boat chase.

　　A ranking system will be included that will essentially let you level up throughout a multiplayer match based on your performance, and each increase in rank will confer a new bonus. First, you'll gain a greater number of grenades; second, you'll be granted a pair of binoculars so you can keep an eye on the enemy; and third, you'll be able to use these binoculars to call in artillery strikes from afar. These ranks will be reset when you quit the game, but it will apparently be fairly quick to ascend the ranks again in a new game.

　　The biggest enhancement to the multiplayer in United Offensive is the addition of player-controlled vehicles that will play a pivotal role in competitive matches. Jeeps and heavy and light tanks will all be available, each of which has its own strengths and weaknesses. Jeeps can move three players at a time, and passengers can fire from the seat or from a mounted machine gun, but of course the vehicle will be especially vulnerable to enemy fire. The light tank houses two players--one of which mans a machine gun turret--and the light tank's main turret can rotate independently of the vehicle's direction. Finally, the heavy tank carries only one player, and its turret always faces directly forward, but the main cannon's firepower is truly devastating, as we saw when it obliterated a light tank with little effort.

　　The original Call of Duty was already a great-looking game, and it's truly impressive that Gray Matter has gotten even more mileage out of the Quake III engine by adding a brand-new particle system that has enabled new effects for debris, weather, clouds, and smoke to be added to the game. Combined with the realistic environments we saw in the demo, these effects make for an even more intense combat experience than what we saw in the previous game.

　　The multiplayer game has been substantially beefed up with new gameplay modes and a slew of new maps.

　　The rain effect we saw in one level was especially effective at both establishing mood and making it difficult to see enemy forces in the distance. It's safe to say that this is one of the best-looking Quake III-powered games we've seen, which is a testament to the skill of the development team as much as it is to the longevity of the engine.

　　Call of Duty: United Offensive may be an expansion pack, but it's cramming effectively a whole new game's worth of content into its $30 box. Between the three new multiplayer modes (accompanied by 10 multiplayer maps) and the 13-mission single-player campaign, fans of the original game's intensely gritty, cinematic war action will find a lot to sink their teeth into here. United Offensive is slated for release in mid-September; look for more on the game soon. In the meantime, check out a new developer interview and a raft of gameplay movies on the game's media page.