After some more thinking I have come up with this scenario which I consider to be the most likely timeline of a large scale war on american soil in the 20th century. It involves a more successfull entente that breaks up after WW1.
My initial ideea required a Russian empire that has survived ww1 relatively intact. (The Ottoman empire surviving too is a nice bonus) Russia needs to stay in/rejoin the entente, so by the time the final peace treaties are signed it must be able to throw its weight around. The Russians and French collaborate to impose a peace threaty that is even harsher for Germany then in OTL. Now it would be quite possible that with Germany out of the picture, the old English-Russian rivalry in Asia is rekindeled. England will be more and more concerned with Russian power and seek to contain it. To do so, it will strengthen the ties with Japan (and maybe the Ottoman empire, if it survives) in the face of the common enemy, and seek other allies (but without much success at first). Eventually Britain will leave the entente, while France will remain loyal to Russia. Tensions between the two rival alliances will grow, and it is probable that Russia and Japan will end up going to war against eachother, dragging their respective western allies with them. Britain will of course be reluctant to fight, but the need to preserve the balance of power wil compell it to act. At first, the Anglo-Japanese, being weaker than their enemies, will be pushed back. In desperation, Britain will turn to a previously unthikable solution: an alliance winth Germany. Let us assume that the Germans, with British help, can rebuild their forces in spite of the inevitable French attack, and that they can turn the tide of the war. The Anglo-German-Japanese alliance must be winning the war, so that they feel emboldened for their next move. We can suppose that during all this time, the US has been happily suplying both sides with war materials. Even close to defeat, the Franco-Russians would still recieve copoius amounts of supplies from America. Obviously, the UK will want to do something about this, but the public will not support yet another war against a fellow democracy. So the English will settle for a "proxy war". They will instruct their German allies to launch an unrestricted submarine campaign against US shipping. It is hoped that the war in Eurasia will be over soon enough and then the UK can broker a peace. Meanwhile, the Royal navvy will try to covertly support the Ubootwaffe, while pretending to stay neutral. However, the pretense of neutrality can not be kept for long. After a series of incidents, thensions between the US and UK rise to unbearable levels. (Let's assume that Japan has allso attacked the US at the same time or at an earlier date, thus creating another source of tnsions). Finally, the Americans can't take it any more and launch an invasion of Canada. The English and their allies rush in reinforcements, and a large scale war rages across North America....
My initial ideea required a Russian empire that has survived ww1 relatively intact. (The Ottoman empire surviving too is a nice bonus) Russia needs to stay in/rejoin the entente, so by the time the final peace treaties are signed it must be able to throw its weight around. The Russians and French collaborate to impose a peace threaty that is even harsher for Germany then in OTL. Now it would be quite possible that with Germany out of the picture, the old English-Russian rivalry in Asia is rekindeled. England will be more and more concerned with Russian power and seek to contain it. To do so, it will strengthen the ties with Japan (and maybe the Ottoman empire, if it survives) in the face of the common enemy, and seek other allies (but without much success at first). Eventually Britain will leave the entente, while France will remain loyal to Russia. Tensions between the two rival alliances will grow, and it is probable that Russia and Japan will end up going to war against eachother, dragging their respective western allies with them. Britain will of course be reluctant to fight, but the need to preserve the balance of power wil compell it to act. At first, the Anglo-Japanese, being weaker than their enemies, will be pushed back. In desperation, Britain will turn to a previously unthikable solution: an alliance winth Germany. Let us assume that the Germans, with British help, can rebuild their forces in spite of the inevitable French attack, and that they can turn the tide of the war. The Anglo-German-Japanese alliance must be winning the war, so that they feel emboldened for their next move. We can suppose that during all this time, the US has been happily suplying both sides with war materials. Even close to defeat, the Franco-Russians would still recieve copoius amounts of supplies from America. Obviously, the UK will want to do something about this, but the public will not support yet another war against a fellow democracy. So the English will settle for a "proxy war". They will instruct their German allies to launch an unrestricted submarine campaign against US shipping. It is hoped that the war in Eurasia will be over soon enough and then the UK can broker a peace. Meanwhile, the Royal navvy will try to covertly support the Ubootwaffe, while pretending to stay neutral. However, the pretense of neutrality can not be kept for long. After a series of incidents, thensions between the US and UK rise to unbearable levels. (Let's assume that Japan has allso attacked the US at the same time or at an earlier date, thus creating another source of tnsions). Finally, the Americans can't take it any more and launch an invasion of Canada. The English and their allies rush in reinforcements, and a large scale war rages across North America....