"Softening Up" is certainly an odd expression.
But a couple of more factors at work:
The Imperialism of the 19th century was fueled not only by economic aggrandizement, but also imperial and dynastic ambitions; World War I not only demonstrated the dangers of total war among the belligerents of Europe, it also saw the waning of such ambitions.
The rise of Nationalism, which at first worked to fuel imperialist expansion, eventually resulted in the rise of Nationalism throughout the world, making the conquered peoples ever more difficult to rule, and eventually led to the so-called "retreat from Empire" after World War II -- an event which not only circumscribed European power, but also promoted nationalist movements throughout Africa and Asia.
Finally, with the advent of a Bi-Polar International system between the U.S./NATO and the U.S.S.R./Warsaw Pact, both armed with Nuclear Weapons, largely changed the nature of warfare in western civilization from "wars of conquest" to "wars by proxy", i.e., largely fighting for "control" of other nations through factions rather than going head-to-head.
A variety of factors, including those already mentioned above, have been at work to change the nature of warfare since WWII.
(By the way, Flatlander. Speaking of Fort Frozen Tundra, every heard of Thule A.F.B.?)