I don't think World War II is necessarily your best bet for this. Rather, you have to have a combination of things to go terribly wrong shortly after World War II if you want to have Western Europe dangerously close to the Soviet sphere of influence.
The best POD for this is probably Franklin Roosevelt not putting Harry Truman on the ticket in 1944. When Roosevelt dies in 1945, Wallace becomes President. The war continues as per OTL, Wallace uses the bomb, but agrees with Stalin to set up occupation zones in Japan much like IOTL Germany. Hokkaido, thus, is taken under Soviet control directly, while northern Honshu becomes the Democratic Republic of Japan. Korea comes under complete control by Kim Il Sung.
Wallace, who is as staunchly anti-imperialist (if not moreso) than Roosevelt, supplies aid to Ho Chih Minh, rather than Stalin. This pisses off the French under Charles De Gaulle. Wallace approves aid for a 'Marshall Plan' ALT, but more aid flows in from the Soviet Union. In Italy, the Communists win the 1948 general election, and in Greece, the Civil War goes in favor of the Communists. France, too, is falling under the influence of the Iron Curtain, as French Communists become increasingly dissatisfied with the flimsy French Fourth Republic.
Wallace comes under increasing attack at home after the Republicans take control of Congress in 1946, and is himself denied renomination by the Democrats in favor of Alben Barkley in 1948. By the time Thomas Dewey is inaugurated, the North Japanese have invaded South Japan and France has had it's own communist revolution at the ballot box, leaving the U.K. isolated in an increasingly 'red' continent.