[UPDATE 6 March, 2009: As mentioned in the comments section, Soviet Assault will be made available for separate purchase for those who already own the original game, as well as packed in as the Complete Edition, which will contain both the original World in Conflict and also the Soviet Assault expansion.]
Before we dive into our play session of World in Conflict: Soviet Assault, a quick history lesson:
2007's World in Conflict was an outstanding game took a very risky step with real-time strategy games, but once the game came out and everyone played it, people found themselves asking, "Why didn't someone do this before?" What World in Conflict actually did was combine the fast pace of real-time strategy games with the class-based, team-based, action of games like Battlefield 2. See, Battlefield 2 was a military team-based shooter where you would choose to play as a certain "class" of character, like a medic, an assault trooper, a sniper, and so on, and you teamed up with other players online who also chose different classes, then moved as a team (assuming no one was being a jackass) to capture key "control points" on the map before the opposing team, which was also composed of players who chose different classes, could snap up all the control points first.
World in Conflict basically took the same basic team-based, competitive concept and brought it to real-time strategy games; instead of playing a medic or a sniper, you might control only a tank platoon or a squad of attack choppers. Basically, when you played as a team, your whole team was like a traditional real-time strategy army with mixed forces (infantry, ground armor, artillery, air cavalry), but since you controlled only one squad, you were free to micromanage the heck out of them and focus on causing as much mayhem and destruction as possible with the game's tactical aids, like the almighty nuclear warhead. (World in Conflict was set in a fictitious war between the USA and the Soviet Union during the 1980s--the real Cold War never escalated to actual battlefield conflict, but in WIC, the simmering rivalry between the two nations becomes an actual war.)
This brings us to World in Conflict: Soviet Assault, an expansion for WIC that was originally planned for consoles, but is, for now, currently planned only for release on PC. Soviet Assault adds the actual Soviet faction as playable units and includes a whole bunch of new toys to play with, including Soviet armor, air vehicles, and units, as well as new tactical aids which you can see in action in this recent trailer. The Soviet units and new tac aids will be available in multiplayer, and will also be featured in the modified campaign. As you may have heard, Soviet Assault will be released as an all-in-one package that includes both the original game and also the expansion (so you won't need to dig out your old copy if you can't find it).
The single-player campaign in Soviet Assault will feature a handful of new campaign missions that you'll play from the perspective of the Soviets. The missions will actually be inserted in between the campaign missions of the original game where they're intended to make contextual sense (rather than a completely separate Soviet campaign). Missions will be broken up by in-engine cinematic cutscenes that show key characters discussing their missions and rallying the troops along the background story of the full campaign.
The early campaign mission we played started off as a night ops mission in the highly contested area of West Berlin, Germany, which is currently staked out by American forces. As the Soviets, your first mission requires you to attack the American encampment, first by sending in stealthy infantry to quietly neutralize guards on foot so that you can take out a handful of antiair guns, then come rolling in with tanks. Ironically, the much-lauded 1989 teardown of the Berlin Wall does still happen in the 1980s in World in Conflict: Soviet Assault, but not as a result of joyous freedom-lovers taking a few swings with a symbolic sledgehammer, but in a hail of Soviet tank fire as your tank crews blast the wall in an explosive fusillade. We pushed forward with our tanks to help capture Berlin and begin the Russian invasion in earnest. In all cases, the powerful and quick camera controls of World in Conflict remain intact in Soviet Assault and let you quickly zoom and pan around the action to get the best view of what has just, or what is about to, burst into flames.
Soviet Assault's handful of single-player campaigns seem interesting, but like with the original World in Conflict, we assume that multiplayer will be where it's at. The expansion (which will be both sold separately, and also packed with the original game as World in Conflict: Complete Edition) is scheduled to ship later this year.