During the Korean War (1950-1953) North Korean invaded South Korea, which was immediately helped by the United Nations (predominately US forces). Once the North was then overrun in a large counterattack, they asked other communist regimes for help.
China answered the call with many troops. However, the United Nations still had overwhelming air superiority. Eventually, the Soviet Union used their 64th Fighter Corps equipped with many brand new MIG-15s and helped to even the odds by fighting in 100s of air battles. The Soviets tried to keep their involvement a secret, even (unsuccessfully) ordering their pilots to not speak Russian during missions. The United Nations however, soon found out about it, but fortunately this never escalated into what could have been the beginning of World War 3.
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By calflier001 (DATANGSHAN AVAITION MUSEUM BEIJING CHINA OCT 2012) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons