Soviet Assault brings a new Soviet campaign that is entwined with the original World in Conflict campaign, tying up the loose ends in the original campaign.

The first mission begins with the final Soviet preparations for World War III, with the invasion and total occupation of West Germany and the annihilation of the US 6th Armored Battalion in West Berlin. Commanded by Colonel Orlovsky, the mission is a success, and the path to occupying West Germany is open. Following this mission, the storyline snaps back to the Soviet invasion of Seattle, 4 months after the invasion of Western Europe.

The Soviet presence in America is, unsurprisingly, met with hostility by the civilian population. Whilst Orlovsky understands their hatred and tries to pursue a more peaceful means of engaging the militia, his subordinate and nephew Captain Malashenko suggests that rounding up civilians to firing squads would lead to a rapid surrender. Orlovsky vehemently refuses such a barbaric notion, and forbids any firing squads. Malashenko ignores this order, and is caught by Orlovsky just before he begins executing prisoners. Angered by his show of disrespect and dishonor, Orlovsky warns Malashenko that if he steps out of line again, he will be sent home in shame.

Going back months before the Seattle invasion, and after the Soviet invasion of West Germany, the situation in Europe has become a stalemate, with very little progress being made by either side. In order to break this stalemate, Orlovsky's battalion is tasked with raiding a Norwegian early warning base, allowing the Soviet Air Force to fly air raids deep into NATO countries such as France and the United Kingdom. Despite the success of the mission, news quickly reaches Orlovsky that NATO forces have made an incursion on Soviet territory near the submarine base at Murmansk. Orlovsky's battalion returns home to defend a POW camp from a NATO attack, successfully repelling them with artillery and helicopters. During the mission, an already disdainful, but patriotic, Malashenko is informed that his wife and child have both been killed in a NATO operation. Infuriated by this, Malashenko states his fury at the lack of progress being made, and the constant lies being fed to the Army and to civilians by the GRU and the government respectively.

Following the aftermath of Cascade Falls, where a tactical nuclear device was detonated to protect Fort Teller, a headquarters for the Strategic Defense Initiative, Lieutenant Romanov, as well as the remaining members of the battalion retreat to Seattle to prepare for the inevitable US counterattack. A flight of Mi-24 Hind gunships are provided to neutralize M270 MLRS positions, but as the mission progresses, Colonel Orlovsky soon realizes the futility of their mission, and makes arrangements for the battalion to return home to Russia. In a fit of rage following this news, Malashenko shoots and kills Orlovsky, and brings his company to Seattle with as many men as he can. Major Lebedjev, the KGB attache to the battalion, assumes command and, following his predecessor's plan, orders the remaining forces home.