Is it just me, or have an unusually high number of Nazi-related ASB-level threads shown up in the last few weeks?
No, they have always been prevalent. But to give the original question the respect it may deserve, what follows are some essential preconditions. Taken together they are pretty much ASB, but here goes:
(1) US does not enter "WW2" and remains more isolationist (say FDR is never elected or loses to an isolationist Republican in 1940, and Japan does not attack Pearl Harbor), US does not enter into lend-lease agreements with Britain - ie remains truly neutral
(2) Germany is able to defeat Britain/occupy and defeats the USSR or neutralizes it diplomatically by maintaining the friendship pact. European war is over by late 1941
(3) Germany obtains control over the bulk of the Royal Navy and French fleet, and efficiently marshalls the production of conquered states to make it true a continental superpower. This would take several years, say by 1945-46. If Nazi-Soviet pact is still inplace Germany never attacks the USSR and the USSR never attacks them
(4) US military attention is focused entirely Japan - but not in a way which leads to war with Germany. Perhaps Germany is able to pull the wool over the eyes of a conservative, isolationist US government so the Nazis are not seen as a critical threat to US security. Perhaps Germany even breaks with Japan and offers a non-aggression pact to the gullible US similar to that they have with the USSR.
(5) A lot depends on the status of Canada and/or Mexico in this TL. Is Canada completely independent from occupied Britain, the outpost of some sort of UK govt in exile still technically at war with the Axis, Under the control of a British govt which is a Nazi puppet, or a US protectorate? Is Germany able to obtain diplomatic inroads (and secret military preparations) in Mexico without raising suspicions in the USA. Basically, it would very much help Germany's chances if they could start with significant forces already in North America without having to stake everything on a cross-atlantic invasion.
(6) Somehow the US government is not convinced to develop nuclear bombs
So, in this TL, lets pretend that, in early 1947, the US is fighting a war in the Pacific, with the bulk of its fleet and army engaged against Japan in east Asia. It has been lulled into thinking German Europe is not a threat while at the same time the Germans have been planning for a massive invasion of North America, making initial use of forces predeployed secretly in Canada and or Mexico. US reconnaisance in the atlantic is minimal. The Nazis strike in a massive sneak attack, invading with relatively small elite forces from both Canada and Mexico and pushing into the undefended midsection of the country, drawing US forces against them, while the bulk of the invasion force is headed across the Atlantic, escorted by the bulk of the Royal Navy, Kriegsmarine, Italian, and French navies. (Formerly British) Aircraft carriers strike at US airfields and naval bases along the east coast. Landings are made along several locations on the east coast, and Nazi troop replenishment continues thru Canada and Mexico. The US immediately sues for peace with Japan and ships its forces back to the west coast, but it is too late to prevent disaster. In several months, most major cities along the east coast have been occupied and over a million German troops are advancing south through the Great Plains from Canada and North from Mexico. They join in Kansas, splitting the country in half, while another force pushes slowly against the middle west from the east. The US federation then begins to disintegrate. The Southern States reform a second Confederacy and sues for peace, which the Nazis grant. United States is reformed in the unoccupied Mountain west and Pacific Coast states The Germans recognize the difficulty of invading the Rockies and eventually, agree to an armistice withthe rump USA.
And there you have it...an invasion of the USA by Germany. Likelihood that this could ever happen = less than 1 %.