When Tony Blair decided to invade Iraq.
I was a Labour activist in the 90s, and I worked hard campaigning to win an important marginal seat in 97. I was a Blairite true believer, fully committed to the idea that social justice & a strong economy were not mutually exclusive. I was incredibly proud of the achievements of the first 5 years of the Labour government, and I still am.
Then Blair lost his mind, sucked up to the odious George Bush and made the disastrous decision to invade Iraq. I felt genuinely betrayed and disillusioned so I quit the Labour Party.