Hitchens was a member of the International Socialists from the mid 60's to early 70's when, as he later said, it was "a small but growing post-Trotskyist Luxemburgist sect". I don't know the circumstances in which Hitchens left IS but it coincided with the transformation of the group into a tightly controlled mono-tendency which banned internal factions and dissent. He subsequently moved to the right, but not as far as some of his detractors on the left claim. Hitchens towards the end of his life said he was no longer a socialist but denied he was a conservative. It would probably be accurate to call him a liberal, albeit a more consistent and principled one than the spineless, hand-wringing types who write for The Guardian. He certainly avoided the fate of his younger brother Peter, like him an ex-IS member, who is now a Daily Mail Tory Anglican caricature.
Even though he was wrong on the Iraq war in 2003, it was for the right reasons (wanting to liberate the Iraqi people from Ba'athist dictatorship) as opposed to much of the left who rightly opposed the invasion for the wrong reasons (anti-Americanism, pacifism or, in the case of George Galloway, friendly relations with Saddam and his henchmen). He was also clear in opposing Islamic clerical fascists like al-Qaeda, refusing to let them speak for a Muslims as a whole and standing up for those threatened by them like Salman Rushdie, in sharp contrast to the IS's successor the SWP who downplayed the threat and promoted organisations like the MAB inside the anti-war movement.
Hitchens' other role was as a debunker of religion. That he chose to do so in the United States where religion pollutes public life to a much greater extent than in most of Europe is to his credit and I think he did it more thoughtfully than others like Richard Dawkins. It was certainly always entertaining, as when he spoke about the death of the charlatan Jerry Falwell in 2007.