Reason: the struggle of countries for political supremacy.
World War I ended in 1918. The Union of the Central Powers was defeated, the empires collapsed, forming dozens of new states. A system of control and regulation of international relations was required. This system was consolidated at the Versailles and Washington conferences (1918/1922).
However, it had many shortcomings: 1. the new system protected the interests of only the victor countries, 2. the predatory regime of indemnity, doomed Germany to re-enter the warpath; 3. the newly formed countries had many internal, national problems. This complex of disagreements led to the fact that the former allies began to fight for power in Europe and redistribution of lands. In parallel, the militaristic countries of Germany, Italy and Japan arose. And the US withdrew from global politics, focusing on American issues.
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