One small comment: Should any Communist American-Faction become dominant in the USA, then would they not rush to their beleaguered Russian allies and declare war on the Japanese?
With the PoD no earlier than '33, that means FDR wins his first presidential election. His VP, John Garner, once said the office of VP wasn't worth a bucket of piss, or somesuch. In OTL '38 and '39, he clashed with FDR over many key issues. What if he clashes with FDR sooner? What if this clash leads to a split in the traditional Democrat party supporting Garner and the New Dealists backing FDR? Would this be a plausible point for violent reactions, especially with the Brown Shirt demonstrations erupting from Europe?
***Say this is the US PoD, and an ensuing civil war (of sorts) begins. Right-wing against Left-wing, or whatever. Fascists vs. Commies on American soil. A new sort of CSA (southern ultra-conservatives) forms and threatens to split the Union again, the war erupts, semi-concurrent with the Spanish Civil War and new weapons and techniques are tested. Mexico eyes the north with something between wild hope and fear, and UK-backed Canada gets a little nervous. This leads to greater debate in Parliament about what to do with the rising Fascists in Europe (and threat of war likely) and the once-allied American Commies gaining support. Eventually the right takes command and a point for Churchill disappears (or somesuch).
The (2nd) American Civil War rages for several years, blahblahblah. Germany and Italy stand firm and go through all the motions of land-grabbing as OTL, though Hitler speaks out against the Bolsheviks in the New World, even offers to send his troops to the USA (Or the CSA? No, not that one, but the Communist States of America?) on the pretext of protecting the (can't think of the word) Americans of German descent/ancestry. Terrarians? Or somesuch.
This alone could be the invasion, if you wish, though unlikely. More likely, Hitler would reach out over the radio to speak to these people and attempt to strike them into some sort of resistance band. Later, these guys could act as a sort of French Resistance that aided the Normandy Landings... just Anglo-German Resistance aiding the... Arlington landings, or something.
With right-wingers in Parliament, that removes the kind old man of Chamberlain from every saying "peace for our time" as likely the Brits would back the German land coups, sticking France's backsides out in the wind. Again, likely, the French would back down, and maybe even go through a puppet Vichy stage.
Russia. Yeah. They're nervous. Maybe help from across the Bering Strait? Move into Manchuria/Manchukuo in a bid to link up with the Chinese Communist Guerillas under some guy named Mao? Maybe even that guy over in IndoChina. What's his name? Ho-Chi-Minh or something. I don't know.
Communist USA emerges victorious in Red Revolution. Mexico is in the heart of its Maximato period, and backed the American Commies. This led to the 2nd Cristero War in Mexico which ended in brutal repercussions amongst that countries catholics, as the country turned more radical and left-leaning. (The Point Here is the Mexico is a likely American-Commie Ally.)
British holds around the world stiffen. Aid is promised to the Japanese, and the Dutch are pressured from both Britain and Germany in to allowing Japanese free trade in Indonesia... or something. British regulars take up positions in Canada. Germany and Italy consolidate the Balkans, angering/scaring Stalin? Not sure here what Stalin would do without the Molotov-Ribbentrop Pact. And without that, how might Finland and the rest of Scandinavia stand in the looming war?
USA begins massive industrialization effort (New Deal on meth) and quickly builds the armed forces with the sciences learned during the Red Revolution. Depending on what is going on in Asia or Europe, the Americans might concentrate in one area or the other. With Brit Royal Navy on one side and Jap Imperial Navy on the other... few easy choices.
Something guy spills his drink on some other guy's lap at a diplomatic meeting and fisticuffs ensue. Smeared over the papers, nations howl for war, and we're off!
American tank columns punch through to Montreal, Quebec, and that other city. You know the one. The Capital of Canada... no one ever remembers the name. Quickly the USA takes control of countryside and the Trans-Canadian Railway and begins shipping material to Alaska where convoys can rush them over to Russia (fighting the Japanese all the way, of course). Germany launches Operation Barbarossa of somesort and nearly takes Moscow. With this, the Russian Eastern theatre nearly disintegrates, but America has begun taking some fight out of the Japs with some allied help (Let's face it, USSR and CSA need some other worldly help besides Mexico and the scattered rebel groups--so who?!). Maybe France kicks up some dust and is a mild annoyance to Germany, slowing supplies sufficiently that Moscow holds through the winter.
Hawai'i falls to a Japanese invasion, along with Midway, and those other places. A joint Anglo-Indian-Japanese invasion of the Philippines (which couldn't figure out how they felt about a Commie USA) puts the Americans on the defensive in the Pacific for a time. British commandos start operation all along Americas coasts, particularly from Caribbean holdings. Italy pisses off the Arabs and the 'modern' Italian army falls on its face a few times in the desert. Germany, afearful of the threat of lost oil supplies, sticks this task to the Brits who are already at hand in those regions. Indian troops have to be brought in for this little maneuver.
And I have to take a pizza delivery, so... yeah, read that and speak further.
***The Communist Revolution of the USA in this thread is not like the American Revolution or the American Civil War, where a definitive fact/ideal/argument is the line in the sand. If anything, it is rather more like the French Revolution, and possibly the Bolshevik, where the leaders in the early stages are not present at the end. Therefore the progression of the ideals and the reasons for the war alter through each subsequent leader. Does that make sense, or am I just a loony pizza delivery boy?