Final answer:
Germany, Italy, Japan, and the Soviet Union were the four dominant military forces in the 1920s-1940s that set the stage for World War II, due to their aggressive expansionist policies and the rise of fascism and totalitarianism.
Step-by-step explanation:
The four countries that emerged as dominant military forces during the 1920s-1940s and set the stage for World War II were Germany, Italy, Japan, and the Soviet Union. During this period, the rise of fascism and totalitarian regimes in Germany and Italy, under Hitler and Mussolini respectively, resulted in aggressive expansionist policies. Japan, seeking to extend its influence across Asia, engaged in brutal conflicts, notably against China, while the Soviet Union, having survived internal turmoil and revolution, also emerged as a major military force. These nations were central to the conflicts that unfolded and eventually led to the outbreak of World War II, with Germany's invasion of Poland in 1939 and Japan’s attack on China in 1937.