Yes, Rishi Sunak, the world is a scary place. That is why we need a new prime minister | Rafael Behr

On the eve of the last general election, no one was debating what to do in the event of a pandemic, or if Russia invaded Ukraine, or the Middle East exploded. If the recent past contains clues about the near future, Britain’s next prime minister will soon be grappling with some as yet unforeseen global emergency. Plus the already visible crises.

Where most people see dangers amassing on the horizon, Rishi Sunak reads simplistic campaign slogans. For want of a domestic legacy to celebrate, the prime minister has decided to put global volatility at the centre of his pitch for re-election. He rehearsed the line in a speech on Monday. The world is a scary place, he said. Only the Tories can keep you safe.

The first half of the proposition is indisputable. Vladimir Putin has burned all bridges of trust between Russia and its neighbours. Israel and Iran, already antagonists by proxy, are a hair-trigger away from open war.

China will continue to rival the US for global influence. Tension between the established superpower and the ascendant one will inflame disputes over tariffs and technology transfers. The US will tilt further towards protectionism. The EU will react by accelerating its pursuit of what Brussels policymakers call “strategic autonomy”. Other rising powers, India principal among them, will game the unsettled balance of global power for tactical advantage and commercial gain.

This all poses unique challenges to a country that recently exiled itself from its local continental power bloc. Brexit was a huge bet against the idea that geography mattered to economic and security policy in the 21st century. Geography won.

Sunak has nothing meaningful to say about that problem. He can only list menaces beyond Britain’s borders and accuse Keir Starmer of lacking the gumption to take them on. The charge has two elements. First, Labour still harbours the Nato-sceptic, pacifist impulses that shaped Jeremy Corbyn’s worldview. Second, the opposition won’t mimic Conservative pledges on defence spending.

The reality is that Starmer has purged the Corbynite left from Labour with a ruthlessness that Sunak has failed to deploy against cranks and maniacs on the right. Naming former leaders who should never have been allowed near Downing Street is hardly comfortable territory for the party of Liz Truss.

As for defence spending, the real difference between Labour and the Tories is that the latter are relaxed about making up big numbers because they don’t expect to have to find the money after an election.

Sunak conjures up billions in future defence budgets by assuming cuts to other departments that no one familiar with the parlous state of public services thinks are feasible. It is the same accounting trick that the Tories use to “fund” tax cuts, all in the hope of trapping Labour in fiscal fictions or exposing it to charges of nefarious intent with public money.

Defence spending will be a priority for Labour in government because Nato needs the money and European democracies need Nato as their insurance policy against Kremlin aggression more than at any time since the cold war. Starmer’s commitment to that cause is not some electoral feint. It is a renewal of vows with the pre-Corbyn tradition of unambiguously pro-western alignment espoused by every past Labour prime minister. (It was Clement Attlee who first took Britain into Nato.)

It is true that any decision to prioritise rearmament will inflame resentment on the left. But Sunak is wrong to think that Starmer and Rachel Reeves will flinch from the task. More likely, they will press on, resulting in a steady drift of disillusioned Labour supporters to the Liberal Democrats (as happened under Tony Blair) and, following the pattern of recent local election results, to the Greens.

The predictable geometric consequence of Starmer governing from the centre is an insurgent opposition on the left rising in symmetry with increasingly paranoid populism on the right. A demoralised, defeated Conservative party will be vulnerable to capture by the faction that sees migration as the root of all national ills and international treaties as an offence against sovereignty.

If Donald Trump is elected US president in November, many Tories will be drawn into orbit around him, cutting what flimsy ties still bind them to multilateral institutions and the rule of law. Neither the current Tory leader, nor any prospective replacement, would resist that tendency. If, as Sunak claims, the question on the ballot paper at the election is how best to safeguard Britain’s democratic values, the correct answer is to banish the Tories to opposition.

What these calculations reveal, above all, is the impossibility of disentangling foreign and security policy from domestic politics. The two have always been linked, not least by the way financial markets, moved by world affairs, can jam levers of national economic policy.

But intensification of global volatility tightens the feedback loop. Russia invades Ukraine and household energy bills spike. War in Gaza brings thousands of people on to the streets in London, demanding things that the British politicians are powerless to deliver.

Conflict anywhere on Europe’s periphery becomes a refugee crisis that animates anxiety about migration. The operation of international law becomes a front in the culture war between liberal defenders of human rights and nationalists demanding border control at any price. And that is before the climate crisis, as a driver of migration and a bill that national governments dread imposing on their taxpayers, is fed into the equation.

It would be wrong to say British politics is ignoring these issues. They were all in Sunak’s speech somewhere, just not arranged into any coherent analysis. David Cameron cuts a serious figure as foreign secretary. He is enjoying the restoration of the stately pomp to which he has always felt entitled. But he is no freer than the prime minister to speak candidly about the chain of strategic miscalculations that have landed Britain in its present predicament.

On the Labour side, David Lammy, the shadow foreign secretary, has written thoughtfully about shifting global power balances and the need to shed complacent illusions cultivated in the era of western hegemony. But it is hard to know what his doctrine of “progressive realism” actually means until it is tested by dilemmas in government.

Some hard choices are predictable – how to handle Trump; how far and how fast to realign with the EU; where to draw the line between rivalry and threat from China. But seeing the questions in advance doesn’t make them any easier. And that is before the next unforeseen crises that will define the second half of this decade just as pandemic and wars have shaped the first.

Amid so much uncertainty, and so little political will to grapple with it, I offer two safe bets: first, the election will not be decided on the question of which party leader has the more credible strategic concept of Britain’s place in the world. Second, that question more than any other will decide whether the next prime minister succeeds or fails.