As per the Insider’s info, I bring you… the grand WG surprise. This will almost certainly be valid for all the servers. Mission starts: 24.12.2014 Mission ends: 15.1.2015 Condition: grind 50k XP for each nation (350k XP total, premium XP and doubles count AFAIK), using a tier 6+ tank Reward: T-34-85M tier 6 premium Soviet medium tank. Each nation (each 50k) is handled by a separate mission. For fulfilling each nation mission, you get 3 days of premium account. After they all are completed, you get the tank.

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