Declinism, the idea that history was headed downwards, progress was reversing, was rampant in the mid-20th century. Not surprisingly, for it was easy to point to the world wars and totalitarian dictatorships as symptoms of decline.
Consider Hans Morgenthau, founder of political realism.* For Morgenthau, a base level of rivalry among states is a constant, but it may be intensified or restrained. In the 18th and 19th centuries, there was restraint thanks to “common moral standards and a common civilization as well as common interests” (222). But this has now collapsed. It is but “a historic reminiscence” so that “only shreds and fragments survive of this system of supranational ethics which exerts it restraining influence on international politics” (258). He likens the situation to “the feeble rays … of a sun that has already set” (258-9) and “an empty sky from which the gods have departed” (259). The blame for destroying the restraints of civilization lies mainly with the crusading spirit of universalistic nationalism.
The problem with this is that there were also countertrends, new restraints on the international power struggle emerging. The decline turned out to be partial and temporary.
*Hans Morgenthau, Politics Among Nations: The Struggle for Power and Peace 3rd edition (1966).