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The Global War lasts for either 8 or 10 days and then you will be able to claim the battlefield rewards based on your performance and afterward you start a new battlefield.
Though it may seem that you have to start a battlefield over and over again, this is actually not a meaningless repetition, as you do earn gold, army rank XP and dark iron points according to your performance in each field. Gold can be used to upgrade units in the Tech Tree so you can always command more and more powerful troops in current/future fields.
XP and dark iron points can improve your profile and world rank. note that World Ranking represents the real capacity of a commander. This game mode can offer a fair start for new players and also allows for an advantage (experience, high tech, great profile) to old players.