Even so if you let them join, it will be weak people vs weak. You should let them join to teach them a lesson. And it's not all about power so what if you water users beat fire, fire is not about chaos and to me water can be peacful at times as well as fire but they both have there moments of chaos.
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Anyway you should let them join if they have learned the water ability. After all you guys need people right, plans, traps skill's, train more make you're skill much better. You guys could well train them in the mean time for the war but it's really up to you, will you take there pride and fight along side them and have them on you're team or call them n00bz and leave it at that. Izuna one more question when i gain the mangekyu sharingan will you fight me, i want to test it on you first, but it will take some time for me to wake up normal sharingan, to me this war does not really matter i may be on the fire side but you're an Uchiha and so am i, and i want to test my power on you once i go threw some painful training. It's not the war that i care about i want to fight you Izuna one day. Who knows we can maybe be friends and sorry for the spam