For WWII Russia to win the need to kill the Master Chief. For Master Chief to win he must kill every single soldier in the WWII Russian Army.
WWII Russia will be at their peak, and armed at their best. Master Chief will have an Battle Rifle, Halo CE Magnum, and will have Jackhammers and Splazers (100 ea) hidden around rural Russia.
So, who wins?
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I'll answer when I know whether Master Chief could beat WWII Finland.