Donald Trump is a pig, a liar, a woman-hater, a racist, a monster of bombast – and did I mention a disrupter and total bastard as well? Does that feel better? Comment on the current US president seems to require a wallow in the pit of competitive contempt. But it just plays his game.

I regard Trump as an aberration, a temporary trauma afflicting US politics. He honours the thesis of the historian Arthur Schlesinger, that America’s constitution often drives the republic to the abyss, only to drag it back again. But even monsters can ask the occasional good question. Thus Trump this week on Nato, a body so mired in platitude and waffle it has lost sight of its true purpose. Trump wants to know what Europe really regards as its defence policy, for he thinks it takes America for a ride. Nato was founded in 1949 in response to Stalin’s blockade of Berlin. It was meant to “keep the Soviet Union out, the Americans in, and the Germans down”. Since then, it has welcomed the American nuclear shield, at vast cost to America. Otherwise, its only military achievements have been the breakup of Yugoslavia and the loss of a squalid 17-year war in Afghanistan. Neither has anything to do with the North Atlantic. Nothing better symbolised this than Theresa May’s bizarre gift to Trump this week of 450 British troops for Kabul.

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Nato was about deterring an attack on Europe from Russia. In 1945, the west agreed the Potsdam settlement, accepting the Soviets’ “sphere of influence” over eastern Europe. Thus when Russia invaded Hungary in 1956 and Czechoslovakia in 1968, there was no question of Nato, or Europe, retaliating. The iron curtain was iron.

Come 1989 and the collapse of Potsdam Europe, Nato did not approach a broken Russia to agree some new settlement. It did the opposite. To protests from Russia’s weakened leader, Boris Yeltsin, it gathered former Warsaw Pact states under its wing and advanced its border east towards Russia. It embraced Poland, Czechoslovakia and Hungary, then the Baltic states, Romania and Bulgaria. It was like Khrushchev stationing missiles in Cuba. Only Germany counselled caution.

Nato’s provocation was so blatant as to be an open invitation to any new populist leader in Moscow to exploit Russia’s bruised patriotism: hence Vladimir Putin. He and his kleptocratic cronies are virtually a Nato creation. But the fact that America was party to the provocation does not invalidate Trump’s question. What is Nato’s policy beyond needling Russia and feebly relying on the American shield?

Play Video 0:34 Trump rebukes European nations over defence spending – video

It is astonishing that, three decades after 1989, Europe is almost back to a cold war with Moscow. As winner of the last war, Nato was primarily responsible for lowering tension and making peace. Instead it revelled in victory. If Europe wants to hire an America nuclear shield, it should deal with America over how to pay for it. But the current tit-for-tat hostilities with Russia are playing with fire, and counterproductive. Europe’s land forces are so weak they would be wiped out by Russia in a matter of days. So is Europe really expecting Washington to order a nuclear barrage against Russian “grey area” incursions into the Baltics, let alone a conflict with Orbán’s Hungary or Erdoğan’s Turkey – both Nato members?

This is not realistic, any more than was American intervention during Russia’s incursion in Ukraine or Georgia. That is why Orbán and Erdoğan are wisely cosying up to Putin. Nato is adrift of realpolitik.

Trump is effectively telling Europe that its Nato is as outdated as the Congress of Vienna by the time of Bismarck. He is wrong to rabbit on about spending 2% or 4% of GDP on weapons. This helps no one but the defence industries – spending should meet plausible threat, not some vague budget target. But no more helpful is Europe’s belligerent posturing towards Moscow, such as Britain’s reaction to the mysterious Wiltshire poisonings. Entrenching Putin behind a siege economy is not a defence policy.

Better to go down the route of detente, recognise Russia’s sphere of influence and be just a little nicer to Putin. Whatever Trump’s motives for advocating this, he is surely right. A sensible Nato would have a firefighting force to handle separatist and frontier squabbles such as Kosovo, not a main force conflict with Russia.

Q&A Can Nato members meet Trump's 4% defence spending target? Show Hide Donald Trump left the opening day of the Nato summit in Brussels in disarray on Wednesday after making a surprise demand for members to raise their defence spending to 4% of GDP. Is the 4% target realistic? The existing funding target – 2% of GDP – is only being met or exceeded by five of Nato's 29 countries, according to newly published figures. They are: the US on 3.6%, Greece on 2.2%, Estonia 2.14%, the UK 2.10%, and Poland on 2%. France currently spends 1.8% and Germany 1.2%. The escalating costs of defence are well known. To build an aircraft carrier, a 21st century fighter jet or a cyber unit capable of defeating Russian attacks is rising at a rate much faster than average inflation. British defence secretary Gavin Williamson has identified a £20bn shortfall in funding over the next 10 years. Without that money, he will need to make further cuts in annual running costs. President Emmanuel Macron, who lopped €650m off the French defence budget last year and sacked his military chief for complaining about it, has promised to boost spending to 2% by 2025, five years earlier than Germany. Where would the money come from? Britain and France have global ambitions, but run budget deficits. That means any extra spending must come from extra borrowing, higher taxes or cuts to other departments. As Trump has pointed out, Germany currently runs a budget surplus and could increase its defence spending to 2% – but doubling it to 4% would be a very long shot. Photograph: Xinhua/REX/Shutterstock/Rex Features

Attempts to set up a European joint force, of which there have been half a dozen since the 1950s, have been fiascos. Britain and France should end their meaningless nuclear deterrents. Their submarines, aircraft carriers and fighters are costly boys’ toys. At present the only role of conventional forces in Europe has been to yield to American blackmail, to join in Washington’s neo-imperial out-of-area wars, mostly against Islam. Trump used to be against these.

Defence planning famously fights the last war but one. Britain’s navy is still fighting the Battle of Jutland and its airforce the Battle of Britain. The money wasted on useless procurement is stupefying. The United Kingdom is perfectly safe from any existential attack: there is no evidence of a Russian design to occupy Britain. Britain needs a decent coastguard, better border security and first-class counter-terrorism. It needs to guard its cyberspace. But its soft power is considerable and its diplomacy respected.

When Britain is over its Brexit crisis, defence relations with Europe will need an overhaul. As a first step, it should start thinking the unthinkable about Nato. For that, thank you Trump.

• Simon Jenkins is a Guardian columnist