With a POD ranging from 1900 to 2020, have foreign troops (a civil war doesn't count) land on American soil...
ITYM
U.S. soil...
Actual "foreign" (Old World) troops on American soil since 1900, excluding troops in colonies (such as the Falklands):
1) German weather reporting detachment in Greenland during WW II.
2) Chinese army instructors in Haiti as part of the UN peacekeeping/civil affairs mission.
Of course there have been thousands of Old World troops in the U.S. and Canada for exercises. Or as PoWs.
So the requirement should be hostile troops in arms, in the Lower 48 or in Alaska or Hawaii after statehood.
OK.
Zangara hits his target (FDR). Garner becomes President in 1933, but fails to check the ongoing economic crash. Food riots break out in some cities in the next year. He orders the Army and National Guard to suppress the disorder, but some units mutiny. Garner's health breaks down. Communist and Socialist rebellions; authoritarian, parafascist reaction. Garner is bullied into naming one Charles Foster Kane, a dictator-wannabe, as Secretary of State, and then resigning, making Kane President. Kane suppresses the Reds by mustering volunteer paramilitary forces. So the U.S. becomes practically a fascist state (the Union), with an active Red underground. The Union's pillars include white supremacy and a generous dose of popuiist anti-semitism.
Meanwhile. the Prince of Wales has his head turned by Lady Rachel Montefiore, who cuts him out of Wallis' clutches shortly after Edward presented Wallis to his family. She's 25, beautifui,
very charming, the maternal granddaughter of Baron Stanley of Alderley, and
she's not Wallis. Edward marries her to everyone's great relief, and hardly anyone cares that she's the great-grandaughter of Baronet Sir Moses Montefiore, the first practicing Jew to receive a British title (her brother is the 4th Baronet). She converts on the marriage, but then her mother converted at marriage the other way. The marriage provokes a flood of insults from the Nazi press, deeply alienating lots of Britons, and some in the U.S. too.
The Union has hegemonic ambitions (taking over all the Caribbean colonies, and annexing Canada).
The U.S. and Germany intervene in China against Japan, as Chiang embraces the Nazi model.
Eventually... global war breaks out in the 1940s.
Germany, Italy, the U.S., and China on one side (the Ring).
(The U.S. is much weaker, due to the internal division between the Union and its opponents. Think massive industrial sabotage, draft resistance, etc. China is actually stronger, with German and American arms and Chiang creating a strong state.)
Britain, France, the USSR, and Japan on the other (the Line).
Throw in minor powers to suit yourself.
At the end of the war, the U.S. is the last Ring power standing, having conquered Canada, the Caribbean, and Central America. British and Japanese forces land in North America to finish off the Union.