How did the anti-war left turn into warmongers?

I explain the strange change in the anti-war left in conversation with Fraser Nelson of the Spectator in a podcast (about 33 mins) and also in the article lined below:

https://www.youtube.com/watch?v=kuo5RhbtsGs

https://www.spectator.co.uk/article/how-the-left-thought-they-were-right-to-fight-the-war-on-terror