World War I drags on into 1920, France, Germany, and Russia fuse into one communist federation. Britain is too weak to intervene due to its own civil war. These communists intervene in civil wars all over Europe, and by 1925 all of Europe has joined this communist state. The United States turns even more oppressive as a result, and you end up with some para-fascist movement taking control. In Britain, "imperial fascism" becomes the main ideology. Due to solid control over Eurasia, the main focus of this nation is building up impressive naval might.
World War II occurs over the communist invasion of China, as Japan tries to protect her interests there. The Anglo-American-Japanese alliance is defeated after a few years, as the Red Flag is raised over London (in part by British communists). Japan is next to surrender, as the nation collapses into civil war between communists and reactionary forces. This gives the communists experience at using communists as a fifth column.
In the interwar period, communists gain considerable strength in Latin America, with several revolutions occuring there, including in Cuba and Guatemala. World War III occurs over these communist states hosting Eurasian military assets. Secret communists agents in the United States (and Canada)--their are too many even for the oppressive government to eliminate--stage strikes the same day as the planned invasion of the US. Guerilla warfare combined with invasions of major ports combined with whole armies moving up through Mexico (the communist side of Mexico's civil war has triumphed) begin the downfall of the United States, a grueling five-year campaign that will cost tens of millions of lives in the largest military campaign the world will ever see.
Basically, any invasion of the US needs control over most of Europe, a substantial fifth column in the US unlike any ever seen before, and many allies in Latin America (probably some Caribbean countries, Mexico and/or Central America, and Argentina and/or Brazil). That just might be enough to defeat the US.
Have a series of cataclysmic natural disasters strike the US in quick succession. The Yellowstone Caldera erupts, a category 5 hurricane slams into the Gulf Coast and a plague infects the Eastern Seaboard.
Then maybe an irredentist Mexico is able to invade the Southwest.
Any plague in the US is affecting Mexico just as hard, and Yellowstone's eruption would cause famine and economic collapse there.