World in Conflict vs. World in Conflict: Soviet Assault

Solution 1:

Found on Steam forums:

World in Conflict and the Soviet Assault expansion share the same game files and folder. You will see the Soviet Assault game menu when launching either game. However, you will still be able to access all World in Conflict levels from this menu. After installing Soviet Assault, starting a new game will allow you to play a campaign that includes the original levels, with the Soviet Assault levels mixed in where applicable. Previously existing saved games will continue to play through the orignal campaign, without the Soviet Assault levels included. If you wish to start a new campaign that does not include the Soviet Assault levels, you will need to rename the file assault.dat found in the folder: C:\Program Files\Steam\steamapps\common\world in conflict\ (default location) and then restart the game.

Solution 2:

The Soviet Assault expansion is a simply number of new missions which have been added through-ought the main World in Conflict campaign. The main campaign of the game concerns an American army unit fighting against the soviets during the cold-war. With the Soviet Assault expansion content available, at numerous points in the campaign you will play a mission as the soviets, fleshing out parts of the narrative, or playing a similar operation from the American missions but from the opposite perspective.

There is no reason to play the original World In Conflict, and as such this feature is not available on the game since it was released on Steam. The expansion was originally included in a re-release of the game on disc and as a free download for existing owners after Ubisoft bought Massive Entertainment from Activision after the merger with Vivendi.