No, it wasn't; Fox had done that before, and and MSNBC, to the extent it eventually did, didn't until much later (9/11 occurred during, and didn't end, the period where MSNBC and CNN were kinda trying to compete with Fox in the same ideological targeting even though CNN was consistently painted as the Left network by the Right, and MSNBC didn't acquire a fairly consistent “left”, or at least Democratic, editorial position until into Bush’s second term, as a gradual evolution from a mix of different editorial positions.
> and when political discourse on the internet started to really sour.
As someone who was online pretty continuously since when “political discourse on the internet” met Usenet, the idea that it started to sour only after 9/11 is silly. More people started interacting online around that time, but that was more driven by easy blogging platforms becoming available and internet access becoming more ubiquitous than 9/11 or any connected political actions.
By the time the post-war occupation had started to slog into a quagmire (the run-up to the 2004 presidential election), we had basically witnessed the dawn of our modern age.