Comments by "Yo2" (@yo2trader539) on "China-Russia Secret Deal Exposed: Is Siberia and the Far East About to Belong to China?" video.
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Japan's alliances with the UK (1902), Germany (1937), and later the US (1951) all had the same purpose, which was to counter-balance Russian/Soviet expansion into East Asia. For the same reason, Japan had cooperated with Poland before WWII as well.
In fact, Japan's war with the Qing Dynasty (1894), war with Imperial Russia (1904), the annexation of Korean peninsula (1910), funding the Chinese Revolution (1911), and Manchuria Incident (1931) were all targeted to keep the Russians/Soviets out of the Korean peninsula and Inner Manchuria (a.k.a China's Northeast Region).
Those were the path of the Mongol Invasion in the 13th century, which is why it was the primary military and diplomatic goal to keep the Russians/Soviets far away from Japan as much as possible. Ever since Imperial Russia seized Outer Manchuria (a.k.a Russian Far East) from the Manchu/Qing Dynasty in the 1860s, it became an existential threat for Japan.
To date, 1/3 of all Japanese military forces are stationed in Hokkaido, which is the closest island to Russia. During the Cold War, the most important Japanese military exercise was to transport/deploy troops to Hokkaido from other regions, in preparation for a Soviet invasion. But unlike in Europe, nobody in East Asia believed the Cold War ever ended.
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