Territory, power and influence. Literally. There are no other reasons. Theo gained too much power and property too quickly and so became recognized as a threat.
Oh okay thx
A little SPOILER warning beforehand just in case......
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when Theo became a lord he gained one territory of the "union" (so he became part of the union) and gained some more territories then the leader of the "union" (that king) tried to take him out because he was gaining too much power, the king (union) lost the battle and asked for help from the "alliance" (the girls adoptive father - because they have like a truce) SO Theo who was planning from the beginning to join the "alliance" was rejected because he is actually "independent" (or a rebel) and if the "alliance" accepted him then the truce between the "union" and the "alliance" would be broken (he needed to be part of the alliance because the territory he really wants to free -his home town- is part of the union so he wouldn't be able to attack his own faction being part of the union)
I hope that I didn't make it more confusing Σ(っ°Д °;)っ
Oh yeah Okay I get it! Also completly forgot that this was his original aim XD
I might be dumb but can someone explain to me why they are at war when they seeked to be in the alliance ?