In order for a ninja to become a Sage one must posses a Rinnegan which is the strongest among the 3 dojùtsu's, and this is a combination of the Sharingan (from the Uchiha Clan) and the Baykugan (from the Hyùka Clan) and a marriage from these clan was forbidden because the child would poses extreme power, not long after the 3rd Shinobi World War the Uchiha's and the Uzumaki Clan, so many people of this clan were dead that the war had to be stopped. Minato was an Uzumaki and thought he wasnt because there was no Uzumaki around him and there was Kushima his beloved sweetheart and not knowing their marriage was forbidden they got married and had a child, how could such child survive such power, and being a Jinchuùriki of the Nine Tails, there must be some truths behind these
There seems to be a lot of wrong information in the question you posted.
In order for a ninja to become a Sage one must posses a Rinnegan
To read more on how the Rinnegan can be activated you can read here.
combination of the Sharingan (from the Uchiha Clan) and the Baykugan (from the Hyùka Clan)
It has not been explicitly mentioned that Rinnegan was a combination of Sharingan and the Byakugan. It was shown that the mother of the Sage of the Six paths had the Byakugan, but that does not imply that the Rinnegan was a combination of the Sharingan and the Byakugan. Infact, to activate the Rinnegan, you don't need to have a byakugan.
marriage from these clan was forbidden because the child would poses extreme power
I don't remember reading anything that specifies that marriage among the uchiha and the hyugaa was forbidden. If you could maybe provide some references then I could elaborate on it.
Minato was an Uzumaki
Karin, Nagato, Kushina Uzumaki, Mito Uzumaki, Naruto Uzumaki
The Third Shinobi War was not fought among the Uchiha and the Uzumaki.
The Third Shinobi World War (第三次忍界大戦, Daisanji Ninkai Taisen) is the third of the four shinobi wars that involved the majority of the shinobi villages. It takes place more than ten years prior to the beginning of the series and has been rarely shown in the series outside the Kakashi Gaiden. Because of a decline in national power, the reign of the Five Great Countries was crumbling. Along their borders, skirmishes with smaller nations broke out all the time. The prolonged war gradually spread its fires far and wide, until at last it developed into the Third Shinobi World War. This war turned into an unprecedented war of attrition, tormenting all nations with a shortage of war potential. Not even excluding a great power like Konoha, very young children were thrown unto the battlefield, losing their short lives.1