Israeli snipers have been shooting at Palestinian protesters over recent days, and Jeremy Corbyn has condemned it. But just a few months ago his spokesperson improbably insisted the Labour leader opposes the boycott, divestment and sanctions movement (BDS), that crucial call from Palestinian civil society which has resuscitated international grassroots action against Israeli atrocities. Those of us who support Corbyn’s leadership require a conversation about the meaning of ‘anti-imperialism’, long a catchphrase for activists, when it is transformed into the policy of a would-be prime minister: when its political subject is not only marginal militants but also a state.

This change in perspective on anti-imperialism is not the only recent shift. On the British left we tend to narrate the destruction of recent decades in parochial terms; we know the miners and the print workers were defeated, but we talk much less about the demise of the ‘tricontinental’. Socialist internationalism cannot now mean what it once did. The old sprawling transnational coalition is gone, which once opposed global capitalism with strikes in its metropole and allied independence struggles in its distant sites of plunder. It was often a flawed coalition, but that matters little now. We have to start afresh, in our anti-imperialism no less than in our quest for domestic roadmaps.

In a strange way though, international politics is just a little reassuring. The German poet Hans Magnus Enzenberger’s doleful verdict on Marx today – “only your enemies remained what they were” – is truer of the grubby, bloody business of geopolitics than it is of the shifting sands of class relations more locally. We have lost plenty of agents of social transformation around the globe, yes, but a British radical offered the reins of state power should have a much clearer idea what to do with the Foreign Office than with the Treasury. There are still imperial wars to oppose and brutal states to condemn: in foreign policy knowing what we are against already takes us quite far, whereas in economic policy the need is much more urgent to build something, and so the left’s intellectual crisis amid the fourth industrial revolution is much more pressing. Honing a distinctive foreign policy should be one of Corbyn’s easiest tasks. He only has to keep saying what he has been saying consistently for decades.

And yet it’s no secret that some close to Corbyn believe a Labour government under him would have its work cut out just renovating Britain’s broken political economy. The responsible, grown-up choice is to clear away the placards from outside the American embassy, on this view; we can’t afford to fight on every front. “Socialism is the language of priorities” said Nye Bevan, key founder of the British welfare state. Labour under Corbyn is not calling for Britain to leave NATO, offer political asylum to Edward Snowden or swap our friendship with Erdogan for aid to the Kurdistan Workers’ Party (PKK). Support for uniting Ireland has gone quiet at just the moment when a hard border risks returning and highlighting the absurdity of partition. Colombian trade unionists and Black Lives Matter activists in the USA have done without Corbyn’s presence at their meetings.

Just how unavoidable are these concessions on foreign policy? They side-step a bitter fight with the parliamentary Labour party, to be sure. Labour MPs are overwhelmingly committed to the foreign policy orthodoxy cemented by Ernest Bevin after 1945, when Britain was America’s debtor and backed most of its gruesome violence overseas. But upsetting MPs with a new tune from Downing Street might be a necessary evil if this is an area where Corbyn can really follow Clement Attlee’s best precedent in changing the rules of British politics for decades to come. More than that, he can redefine Britain’s national identity. Amid the steep challenges of overhauling a whole economic model, in foreign policy a mere declaration – imposing an arms embargo against Israel now, say, or even making good on the overlooked ‘S’ in BDS – can upset the international order and give other governments the confidence to join the charge. These principled positions can also win votes. Even without being naïve about the likely backlash, this is still among the easiest places to build a genuinely radical legacy. Swedish prime minister Olof Palme decried napalm, apartheid and Franco in fiery terms and lent state support to freedom fighters everywhere. Corbyn could do the same, and from an even more prestigious perch in Downing Street. He should be bold.

Popular tales of ‘electability’ and what it means to be ‘prime ministerial’ caution against such things, but these are contortions that tell us more about the ideology of imperial statecraft than they do about real political possibilities. These ideological presuppositions run very deep and make some quite simple changes of policy look unimaginable for the British state, but it really is worth tackling these demons. When Corbyn began talking about Britain’s weapons sales to Saudi Arabia he broke a silent set of rules demanding that British politicians should not be forced to defend dipping their hands in blood in public. His stance proved popular. When he responded to the Manchester terrorist attack with searching criticism of Western foreign policy that seemed popular too; he said the same sorts of things he had been saying for a very long time, but after the long irrelevance of the left it all sounded refreshing to a public accustomed to hearing nationalist platitudes from its leaders on these occasions. Labour’s ‘moderates’ were appalled. The lessons of these two moments are not generally enough applied by the Labour leadership, still too cautious and beholden to the myths of stately propriety when authoring their foreign policy. The shadow foreign secretary insists Corbyn is a ‘Zionist’, and she keeps her job.

Foreign and domestic policy were once entangled in the minds of the revolutionary left. Anti-imperialism was a strategic choice, since at the start of the twentieth century it seemed global capitalism might be brought down by a two-pronged assault in Europe and in Europe’s colonies. That’s not the politics open to Corbyn now. Conditions have changed too much. A properly Corbynite foreign policy would sound more like the radical flank of the old liberal anti-imperialism, a moral protest from government rather than a tactic for revolution. And it would be more consistently anti-imperialist than some on the left; always knowing which side of a picket line to stand on, Corbyn supported workers in Solidarity against the Stalinist overlords who once ran Poland. Now he must know that the cynical rhetoric of ‘counter-terrorism’, when it comes from violent states crushing people who seek freedom, is abominable whether authored by Assad or by Netanyahu. Palme condemned the Soviet tanks in Prague no less than the American planes over Vietnam, and rightly so. Britain will be able to hold its head high in the world when it refuses to take orders from Washington or from Moscow, and finds its friends instead among those working everywhere to humble the powerful and extend that precarious realm of human dignity.