The first postwar "Imagine if..." dramatizations of the Russians conquering and enslaving America, Is This Tomorrow? was published in 1947 by the Catechetical Guild Educational Society of St. Paul, Minnesota. At ten cents a copy, this fifty-two page, full-color comic book was a smashing success. It enjoyed several reprintings, and was used as a giveaway, presumably distributed to church groups. Some four million copies were printed.
Feverish Commie-takeover scenarios emerged in the mass media in the years to come, including Life magazine's "The Reds Have a Standard Plan for Taking over a New Country" (1948), the M-G-M cartoon "Make Mine Freedom" (1948), Columbia Pictures' 1952 film Invasion USA, the 1962 TV special Red Nightmare ("presented by the Department of Defense"), and such comic books as "The Sneak Attack" in the first issue of Atomic War (1952). But none of them could quite match Is This Tomorrow? for pure holy terror.
For more materials of this sort, see my 1950s site.