The Apocalypse is a phenomenon slowly taking effect over the course of the events of Wolfenstein: Youngblood, and a good portion of time before that.
Upon reuniting with his two daughters, B.J. revealed that before dying to his hands, Hitler ordered the construction of a weather manipulating doomsday device. Knowing full well B.J. was going to come and take his life, Hitler programmed a fail-safe protocol in the device which would activate upon his demise, so he could take the world down with him.
After B.J. killed him, the device activated and it was too late to reverse the process. The device caused the weather patterns on Earth to behave erratically, with freak thunder storms, tsunamis and numerous natural disasters. Newspapers such as Die Neue Wahrheit produced in Germany often deny the nature of the apocalypse, claiming that "Aryans aren't afraid of a little weather" while Nazi meteorological organizations have attempted to calm a frightened European populace by declaring a new fifth season based off the apocalypse, called Sturmzeit. Some factions within the Reich, apparently aware of the nature of Hitler's doomsday device, have plotted to topple the Berlin regime in order to stop the impending apocalypse by evacuating the Reich to another dimension using Da'at Yichud technology. Based on Set Roth's calculation, in the next few decades, the Earth would not be able to sustain human life any longer. As the result, the God Key is utilized to create an inter-dimensional gateway to evacuate mankind to another world to survive.
In actual history, Adolf Hitler issued the so-called Nerobefehl or "Nero Decree" on 19th March 1945, once it was abundantly clear that Germany could not win the Second World War. The decree is a scorch earth tactic called for the destruction of all infrastructure and resources by retreating German forces; if put into effect, it would have utterly destroyed what was left of the German economy and caused massive hardship and mass death among the German people. The man in charge of the directive, Albert Speer, Hitler's Minister of Armaments, disobeyed the orders and encouraged other military and political leaders to do the same, sparing Germany's people even greater hardship than that inflicted by their catastrophic defeat.
Hitler was also noted to have said that "If the War is lost, then so are the German people. True Germans will die before surrendering; those that do not fall in defense of the Reich are not worthy of their Aryan blood."