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The only moment that could truely be considered the cause of the war of five kings is the true king's death, but really the entire cause of the war was Ned's actions leading up to his death. Had Ned not decided to tell every untrustworthy person that he'd discovered the incest and failed to tell Robert the war of five kings would have not happened or at the very least would have gone differently. Had he not told Cersei she wouldn't have caused Robert's accident, had he made it a thing before Robert died then Cersei and the kids would have been fucked and at the worst it would have ended up a war of two kings with the Starks with Stannis and the Lannisters with Renly. Had he not told Roberts brothers they wouldn't have had the basis of their claims. Had Ned not been killed Rob wouldn't have tried the king in the north nonsense. There's also the possibility that if Ned hadn't told Cersei, Littlefinger would have killed him before he could stir shit up, that seems to me like Littlefinger's best option before Cersei started her scheming.

To MDK's theory: Preventing the assassin and Cat kidnapping Tyrion wouldn't have prevented the war, it might have prevented the war in the north from starting so soon, leaving Tyrion and Jaime to make the decisions in king's landing during Ned's investigation and Tywin showing up immidiately after Jof became king, leading to things being much better for the Lannisters. That whole plot line was mostly unconnected to the plot line that caused the war.

To Jorick: Changing either of the things you listed wouldn't just prevent the war of five kings plotline, they would prevent the entire series, it is best not to think in those terms.

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