here is how it happened. more detailed.
The film's backstory involves several alternate history political precedents. The Green Party came to power in West Germany, forcing the removal of U.S. forces from that nation and all nuclear weapons from Europe. The resulting upheaval left NATO as a political nonentity, with only Britain remaining as a U.S. ally. At the same time, Soviet allies Cuba and Nicaragua each expanded their armies to 500,000 men, subsequently overrunning El Salvador and Honduras. A civil war in Mexico resulted in that country falling behind the Communist "Iron Curtain." In a parallel to Operation Barbarossa, the Soviet Union, like Nazi Germany, now had a broad base from which to invade its primary enemy, and thousands of troops from satellite nations to augment their own armies.
During this time, the Soviet Union was suffering its worst wheat harvest in 55 years and food riots were occurring throughout the nations of the Warsaw Pact. Apparently desperate for food to feed its people, the Soviet Union and its allies launched a full scale invasion of the United States. Although the movie was released in 1984, the story itself takes place in the near future, starting in 1989 and ending in 1990, because the Holodomor of 1932-1934 is probably[original research?] what is being referred to in the opening narration as the previous "worst wheat harvest" that happened 55 years beforehand.
The Soviets utilize a three-phase attack. First, they use strategic nuclear strikes to destroy key points of communication including several major U.S. cities (Omaha, Kansas City and Washington, D.C. are specifically cited). Strategic nuclear weapons are also used to destroy ICBM bases in Montana and the Dakotas. In addition, it is hinted that Cuban infiltrators aid in confusing U.S. forces by raiding Strategic Air Command bases throughout the Midwest and Texas. Coupled with these nuclear attacks, Soviet transport aircraft containing elite Soviet VDV and Cuban paratroopers slipped through the U.S. radar disguised as commercial airliners.
The second phase saw Cuban and Nicaraguan armies (with small contingents of Soviet forces) pouring across the U.S.-Mexico border into the Great Plains of the United States. The third phase involved a Soviet invasion of Alaska across the Bering Strait from Siberia. They crossed into Canada occupying the Yukon, British Columbia and Western Alberta, most likely including Calgary[original research?], and cut the Alaskan Pipeline, but were decisively stopped at the U.S.-Canadian border by U.S. forces (thus preventing a link-up with Soviet forces occupying the Great Plains).
Elsewhere, Britain remained loyal to its American allies, but suffered heavily for it. China also declared war upon the Soviet Union, however incurred significant losses, Tanner claiming that there were only "600 million screaming Chinamen". When asked "I thought there were a billion screaming Chinamen?", he cryptically replies "There were" and throws his drink in the fire, causing it to flare and suggesting China had suffered Soviet nuclear strikes. In the film, the effects of the nuclear weapons are not shown because the location (northern Colorado) is far from any targeted sites.
The Communist forces manage to occupy and control a large region of the central United States, extending as far west as the Rocky Mountains, and north to Cheyenne, Wyoming, across Kansas to the Mississippi River in the east. Denver is also under siege. Tanner mentions that conditions in Denver are particularly bad, with people living on "rats, sawdust bread, and, sometimes, each other." He says the remaining holdouts most likely won't survive.
Once the lines are stabilized, it quickly becomes a conventional war with both sides ceasing their use of nuclear weapons. Colonel Tanner explains that the Soviets are reluctant to use any more nuclear weapons, as they want to conquer the United States, not destroy it utterly, and the U.S. government is unwilling to use tactical nuclear weapons on or over their own soil against the invading armies.
