If another nation usurps a great power's status, that state will have six (6) months to recover its position before losing its Sphere of Influence. The usurped power is also given a Casus Belli and the option of starting a war to regain their great power rating. If all these efforts fail, then the usurper will become a Great Power and the former power falls to secondary status.
Initial Great Powers
The following nations are Great Powers at the start of the game: