That would be "operation werewolf", the 1944 plan for SS to form a resistance behind allied lines.

Arguably there was in fact SS guerrilla activities up until 1947 as detailed in the book "The Last Nazis: SS Werewolf Guerrilla Resistance in Europe" by Historian Perry Biddiscombe. But any resistance post ww2 never really grew any more severe than semi nomadic SS fanatics living in mud huts far from people, with little or no popular support. The allies categorized them as an insignificant threat, despite a number of assassinations and sabotage operations carried out by werewolves.

They never had any real popular support because Germany as a whole suffered a war-weariness it is hard to imagine today, and were more than happy to cooperate with the allies. especially considering the allies were the only military force that could protect them from Russian war machine.

We actually had an April's Fool thread about that this year here where several flaired user including myself gave answers in character.

In the end a couple of factors played a role:

• Manpower: Contrary to for example Yugoslavia, there was not a huge number of well trained German soldiers who managed to escape Allied captivity and thus hardly anyone able to lead such an uprising.

• Bad preparations: The werewolf training program from 1944 did not fare very well because they were underfunded. Financial and man power resources at that time went either to the Wunderwaffen or to the front. There were not many people able to train people nor were there many able bodied fighters to be trained.

• The convictions of the political leadership: Hitler believed that if the Germans lost Berlin and the war, they were doomed as a race. He and a lot of people in the political leadership believed that guerilla warfare was not a useful tool because once the war was lost by conventional means, it was lost totally.

• Lack of leadership: With Hitler dead, there was by 1945 no political figure left to rally around. Himmler was not popular enough and everyone else had either been dismissed by Hitler or grown to be highly unpopular like Goering.

• Allied tactics: The total occupation and the way the Allies fought, the German were war weary. There simply was not means to gather resources for guerilla warfare and the population also felt that the way the war ended, it was truly over with total occupation.

As I mentioned in my answer in the linked thread, the only really dangerous case was the Soucek-Rössner conspiracy in Austria but that quickly broke apart over fights who would be the "Führer".

Sources:

• Perry Biddiscombe: The Last Nazis: SS Werewolf Guerrilla Resistance in Europe 1944-1947. Tempus Publishing, London 2004.

• Volker Koop: Himmlers letztes Aufgebot: die NS-Organisation Werwolf. Böhlau Verlag, Wien-Köln-Weimar 2008.

• Klaus-Dietmar Henke: Die amerikanische Besetzung Deutschlands, München 1995.

• Hellmuth Auerbach, Die Organisation des "Werwolf".

According to Kershaw, by war's end the conflict had depleted the supply of effective manpower, materiel, and morale that would have supported the extension the war past the point of political surrender. The German people barely tolerated the continued conflict (it's actually remarkable that they tolerated it to the point they did) and had themselves been conscripted by the Reichsgau to dig trenches and build fortifications for an anticipated defense of the Reich. The Volkssturm (the Nazi version of the British Home Guard) had pressed into service nearly every available man who had been found unfit for conscription (too old, too young, injured or otherwise infirm) for anticipated rearguard actions.

The public's willingness to be conscripted into such bodies had less to do with continued allegiance to the Reich and more to do with widespread angst over the possibility of occupation by the Soviets. Rumors (and truths) of Soviet reprisals for Nazi war crimes had led to panic across the reich, including mass suicides in areas close the the Eastern Front, most famously in Demmin where an estimated 2,500 committed suicide rather than face occupation by the Soviets. Indeed, it was the attitude among most Germans that occupation by the United States was preferable to occupation by the Soviets, and as such resistance to American occupation was sporadic, limited and ineffective.

There was also Werwolf, a planned stay-behind operation meant to commence a distributed, leaderless guerrilla war against an occupying force. The plan failed to advance to the operational stage - a number of weapons caches were buried (though few have been located) and money/equipment allotted, but the operation largely failed to exit the runway.

Fun historical sidenote: Werwolf was intended to operate similar to what we think of as terror cells today - autonomous, leaderless units planning and conducting missions of assassination and sabotage on an occupying force. As for the Nazi government, Hitler never seriously intended to leave Berlin, even in the event of Soviet occupation - he had always intended the reich to fight to the last, no matter how unrealistic a prospect that might seem to us now. Nevertheless, Goebbels concocted a scheme to worry and distract the Allies: the Alpenfestung, or Alpine Fortress, a fictional national redoubt in the mountainous regions of southern Germany and northern Italy where the remnants of the Reich, backed by massive numbers of men and materiel, continued the war against the Allies. It was all a myth. Goebbels used the Abwehr to disseminate rumors about the Alpenfestung including fake blueprints and vaguely menacing reports of unit readiness within the Redoubt which were leaked to Allied intelligence in order to distract Allied forces as they crossed the Rhine and were forced to choose between a narrow front focused on taking Berlin versus a broad front stretching from northern Germany all the way to the Alps. A broad front would have significantly stretched fighting strength in the final phase of the war and meant huge casualties for the Allies. The distraction almost worked. Huge embarrassment for Allied intelligence.

From an operational perspective Werwolf failed to achieve its stated ends except in a handful of instances where fanatics and SS hardliners staged individual actions against occupying forces (a handful of assassinations, isolated attacks on troops, usually in the form of traps, bombs etc). Werwolf failed in part because it was too little, too late, and it failed to anticipate either the depletion of military materiel and morale or the public will to continue the war that would have sustained a stay-behind operation. Werwolf instead served an ancillary purpose by coordinating and facilitating underground escape routes ("ratlines") for NSDAP VIPs, a purpose that existed cozily with Operation Paperclip, the post-war Allied effort to expatriate useful human assets (chiefly scientists, engineers and political figures with intelligence to offer) from Germany.

It's arguable that had the war "ended" sooner, with reserves of manpower and materiel more intact, that a stay-behind action would have been of greater effect. Consider the challenges faced by the American army in occupying Iraq after the initial phase of Operation Iraqi Freedom. The war itself lasted a matter of a few short months, with the resistance to the occupation being the main and most bloody obstacle to the occupying army. This is very much what Werwolf might have looked like (albeit on a much larger and more troublesome scale) had the war ended early enough that there were enough motivated, armed personnel with enough guns, ammo and explosives able to melt back into a motivated, radicalized populace and become a hidden opposing force. The war might have looked a lot different.

Edited for clarity, spelling, punctuation and a minor fact check.