One of the most frustrating things about Jeremy Corbyn's opponents in the Labour Party is their gradual acceptance of his value system. They concede far too much. Thus Corbyn isn't fundamentally wrong, he's simply failing to hit the target. Like the enthusiastic but middling player who graces every Premier League football team, Corbyn simply lacks the requisite talent and polish to perform at the highest level. Credit where it is due, though – he does mean well, he puts a shift in and is, at bottom, an 'honourable and decent man'.

You get the drift. This goes some way to explaining why Corbyn's challenger for the leadership, the otherwise sensible Owen Smith, is now waffling wildly about sitting 'round the table' with Islamic State (Isis). Though I cannot be sure, I suspect Smith knows that even entertaining such a strategy is vacuous nonsense. What, after all, would Britain get round the table to talk about exactly? What common ground exists with IS? If they stop threatening us, will we allow them to hold on to at least a few of their sex slaves? Are they to be permitted to carve off at least a small part of Syria and Iraq for their hellish and totalitarian 'caliphate'? Is the best response to murderers and sadists really to grant them even a modicum of legitimacy?

This, though, is what 'widening the debate' – the woolly justification given by those Labour MPs who nominated Corbyn for the leadership last summer – entails. We are now having to talk seriously about the prudence of sitting down for cups of tea with IS and abandoning NATO at a time of increasing Russian belligerence.

In practice, 'widening the debate' has meant dumbing it down to include the sorts of views that are confined to the periphery for quite a good reason. As anyone with a cranky relative who mutters about 'controlled explosions' whenever 9/11 is brought up knows, it is quite possible to be so open-minded that your brain falls out.

But this is what happens – and what will keep on happening – if Corbyn's opponents purport to be even more faithful Corbynistas than the man himself. You won't out-Corbyn Jeremy Corbyn – and nor, on principled grounds, should you even entertain it. The barbs directed at Corbyn by his Labour opponents – overwhelmingly dwelling on the fact that he will prove an electoral catastrophe – are true enough, of course; however, they smack more than anything of political cowardice. The problem isn't simply that Jeremy Corbyn is unelectable. The problem is that his apparently unassailable 'principles' stink like a carton of rotten milk.

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To recognise as much isn't to sell out, or, to use the preferred online echo-chamber vernacular, to 'shill for the establishment'. Nor should it mark a person down as 'right-wing' in any normal sense of the term. When Corbyn recently addressed the Cuba Solidarity campaign, or a bit further back-pocketed money from Iranian state television, he trampled all over the principles of genuine leftists who spend their lives fighting dictatorship rather than pontificating about 'a better world' in the cafes of Islington and Hackney. There is nothing brave or principled about betraying the very society that permits you your eccentricities on behalf of those that do not hesitate to throw their own dissidents into dungeons.

The same holds true with regard to Corbyn's attitude to war, a subject over which the word 'Iraq' seems to provide cover for everything from wet-behind-the-ears naiveté to naked acquiescence in the warmaking of undemocratic foreign powers. A desire to avoid war is admirable; a willingness to walk by on the other side at every instance of aggression and genocide is as immoral as any of the 'America First' tripe emanating from the mouth of Republican presidential nominee Donald Trump.

To paraphrase Leon Trotsky, you may not be interested in war, but war is interested in you. Or as the democratic American socialists Irving Howe and Stanley Plastrik put it during the Cold War, socialists cannot 'retire to some isle of rectitude more or less equidistant from both sides'.

The Labour movement needn't cheerlead every one of America's overseas adventures, but it should, in Howe and Plastrik's formulation, act as the 'socialist wing of the West' – which implies at the very least that it recognises its allegiance to the West in the struggle against dictatorship and tyranny. Should Russia decide to invade a NATO country, it doesn't constitute warmongering to wish to defend an ally.

Criticism couched safely in language around his being 'unelectable' does nothing to drain the foul swamp so as to slay the monster lurking at the bottom. 'We're all Hezbollah now', declared Corbyn's friends in the Stop the War Coalition a decade ago. If Corbyn's 'principles' carry on getting a free pass from his Labour opponents, before long we'll all be making stupid calls for amiable cups of tea with IS.