Quicky ;-)
Point of departure? I have a scenario where the US goes to war with the allies in 1913 over Mexico, Germany and A-H joins the allies in 1916, USA faces revolution in 1918.
In 1919, allies humiliates the USA with the Toronto treaty. California is returned to Mexico, secession of southern states and New england, interdiction for more than 250 000 troops etc.
A Nationalist party arise in the rump USA, put the economy back on track and rearme.
In 1938, they launch a war to reclaim all lost US territories. Greater Germany joins the allies once more when the US have conquered North America and is invading South America.
Bulk of US troops in South America are contained then cut-off when British/German navies retakes control of the seas and launch major operations to recapture Panama, Venezuela & Colombian coast.
The 4 followings years sees the liberation of North America from US influence.
In April 1945, south and central american troops are besieging Washington.