 If you liked
Kidou Senshi Gundam SEED
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 ...then you might like
Buddy Complex
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Buddy Complex and Gundam SEED involves a war between factions in a sci-fi environment. Both series' main male protagonist becomes a pilot of a gundam/mecha to fight for what they believe in.
There is a complexity of supernatural elements involved in both series that makes a big difference in the war. Plenty of action is offered as mecha warfare takes the skies and power struggles become a central struggle between the allied nations.