I'm disgusted with you Huw to be honest. You're saying things like "Corbyn's gone on stage today and told folks that the terrorists are right" and that he's "using" ie. exploiting a terrorist attack - but as a campaigning Lib Dem, you know full well he's not saying anything that isn't entirely consistent with his entire history as a politician. What you're saying here about this is every bit as unfair as Boris Johnson saying Corbyn attempted to "justify" the bombing. He did no such thing!
In fact, he explicitly stated that the blame can only lie with the perpetrators. Repeatedly.
As you're probably repeating such sentiment to other people, I'd ask you to reconsider this actually.
To posit the possibility that our misadventures in places like Iraq and Libya only make the threat greater is no crime, in fact, it is a virtue to be voicing such thoughts at a time when politicians - feeding off of understandable public passion - might seek to curb our own rights, or wage a punishing response on some far flung land and a people, many of whom will have had nothing to do with it. It is the responsible thing to say and this just further enshrines Corbyn as someone willing to say the kinds of things that others in the political elite will not when it comes to military action.
We latched on to the French and American led intervention in Libya, possibly because we saw in it a chance to foster the Arab Spring, and a chance to remove Gaddafi - but just like Afghanistan, just like Iraq - the planning for the aftermath wasn't there and we've left much of the country a wild west for feudal militias. It's one of the conduits via which people drown trying to desperately reach European shores. What did we really achieve and how did it further British interests? How, after three wasteful military campaigns, are we any better placed to deal with a situation like Syria and IS? I'm sorry, but the stalemate there between the American/Western supported groups and the Russian backed regime is something we probably shouldn't aspire to get too deeply involved in either.
As for where negotiating can begin and end -- obviously you can't negotiate with people who join these groups purely to annihilate people in suicide bombings, but you can isolate and influence people, stem the flow of resources and people, cut off money, incentivise people to leave such groups and down their weapons, offer an ideological alternative and hope of a different reality. Do you honestly think Syria is going to be a warzone forever? That it's going to be this way until every even remotely sympathetic daesh supporter is put beneath the ground? As with all conflict of this sort, it will rumble on until circumstances will allow it no longer. Until people are sick of the death and dying, and until people can learn to hope and live for the sake of living instead of just living for the glory of dying.
How in fucks name does some kid - a young adult - go from being born here, his family having found refuge here, go through our educational system, having friends and family and a life here - only then to go down some kind of lost, dark path where he ends up bombing children? I mean, there's no right answer to that question. There's no reasoning the unreasonable. The mere existence and foregrounding of this violent, insane ideological strain of thought - the mere fact that things like this happen - it's always going to attract susceptible minds, and if those minds have the education and the means, things like this can and will happen again.
The debate should be about how we minimise the risk of that, and Corbyn is absolutely right. We do that through being a responsible actor on the world stage, and by investing in the defense and intelligence apparatus, as well as the public services that keep us safe from harm, that prevent and prosecute serious crime, and that keep us healthy - both in body and mind.
Foreign interventionism as embodied by the almost unilateral "war on terror" has been a failure and it has made the world a much more dangerous place. We wasted trillions of pounds on it while allowing our infrastructure and societal cohesion to crumble and buckle. We can build a better Britain and a better future for Britain, but we need to start prioritising the things that matter to make that happen.