In 1917, the Bolsheviks seized power in major Russian cities in a coup not supported by the majority of the Russian people. They fought and won a bloody civil war. Famine savaged the people. The new government repudiated the debts to foreign investors in Tsarist government bonds and seized foreign property. The new regime sought to export revolution all around the globe. As a result, Communist Russia spent many years as a pariah country among the nations. The new Soviet Union had little trade, no foreign investment, and constant harassment of its foreign operations. Then came the collectivization of agriculture, more famines, and the purges. Then Nazi Germany attacked. A commonly-accepted estimate is that the war killed 20 million Russians. Nevertheless, by the end of the war, the red flag flew over the Reichstag.
It’s certainly possible that the Russian people have gone soft in the decades since the collapse of the Soviet Union. Perhaps failed hopes for a swift victory, hundreds of combat deaths, tough—but not complete—economic sanctions, near-universal moral condemnation, and scathing satire on late-night television will bring Russia to its knees. Or perhaps the badly out-gunned Ukrainians will suffer grave defeats, many additional millions of refugees will flood across the border into the European Union, and Russia will confiscate Western businesses and turn off the gas to Europe as further bargaining chips.
The peace settlement following the First World War left many people disappointed or unhappy. Germany suffered territorial losses, had to agree to pay heavy reparations, and had its sovereignty limited by disarmament. At Versailles, “Meester Veelson” and other progressives denied Italy the territorial gains promised to it in exchange for entering the war in 1915. Japan found its wartime efforts at empire-building in China checked. Russia lost vast chunks of territory through the Western-sponsored triumph of national self-determination. While German grievances won substantial redress during the Twenties, Italy and Japan remained dissatisfied.
The Great Depression transformed international relations, just as it did in domestic matters. The conventional economics of the time commanded budget cuts and military spending suffered. Democracies turned toward domestic reforms, or became paralyzed, or just collapsed wherever it had failed to sink strong roots. In the decidedly un-capitalist Soviet Union, Joseph Stalin had seized to power in the late Twenties. The Depression brought Adolf Hitler to power in Germany, while the government of Japan increasingly fell under the control of imperialist soldiers. Italy’s Benito Mussolini ranked as the senior dictator in the group, although he had little yet to show for his tenure in terms of expansion.
The democracies were hard put to deal simultaneously with three open and one covert aggressors. In the Thirties, the United States turned from partial engagement in international affairs to a deepening isolationism. France’s interests and resources were almost entirely European. Britain had the means to fight one war, but faced enemies in the Far East, the Mediterranean, and Central Europe. Things had to get much worse to change policies.
Today, Russia, radical Islamists in the Middle East equipped—or almost—with nuclear weapons (Iran, Pakistan), and China all can be counted as dissatisfied powers. Where to start?
 Nicholas Riasanovsky, A History of Russia (1977) is cold and comprehensive.
 Raymond Sontag, A Broken World, 1919-1939 (1971) remains a valuable guide.