Donald Trump is “appalling and unhinged”, says everyone. His invitation to Vladimir Putin at the weekend to invade Nato and “do what the hell he wants” if Europe does not spend more on its own defence, “puts all our security at risk”, warns Nato boss Jens Stoltenberg. The alliance is supposed to be the bastion of liberty against dictatorship, not about who pays for what.

We might start by pointing out to Trump that the most outrageous abuse of Nato has been by the US. Washington’s demand that the alliance support its retaliatory invasion of Afghanistan after 9/11 was a costly, drawn-out fiasco that had nothing to do with western security and everything with American neo-imperialism. Likewise, its summoning of allies to join other militaristic interventions, from Vietnam to Iraq and across the Middle East. The US’s “withdrawal from empire” is proving as bloody as was Britain’s, and if anything, more ham-fisted. For a former US president to incite Putin to war on Nato is outrageous.

So Trump is indeed unhinged, but does he have a point? He is consistent. For the past decade he has encouraged one of the US’s periodic bouts of isolationism. He has suggested that, with communism dead, the idea of ideological empires in global contention is outdated. To him, western countries have troubles enough at home. It is not their job to intervene in other people’s border disputes or internal conflicts. Britain and others may itch to play a role on the world stage, in the style of the 19th and 20th centuries, but if so they should pay for it themselves. Let them build stupid aircraft carriers that don’t work.

At a more pragmatic level, the turn-of-the-century expansion of Nato to include the Baltics and Poland was blatantly provocative. Putin’s response in Ukraine was so shocking the west was right to help rebuff it, having left alone Putin’s lesser antics in the Caucasus. Now rebuff has turned to stalemate and some way out must be found. Nato must become a force for peace, not endless war. If it does not intend to fight for ever in Ukraine, its long-term intentions remain unclear. Trump’s scepticism is justified.

It is hard to believe that Trump would actually disband Nato, as he has reportedly privately threatened to do. But the alliance’s article 5, its pledge to go to war if any member’s integrity is threatened, is looking in need of reconsideration. This is especially so in the light of the US’s tilt towards the Pacific, and Europe’s own dwindling sense of political unity. It is hard to believe Putin would try to topple more governments across eastern Europe. But he might well disrupt borders and stir Russian minority discontent in frontline states. Georgia 2008 or Ukraine 2014 are more likely precedents than Ukraine 2022.

In that case Trump – and not just Trump – is entitled to ask what business is this of the US. It is not clear that Europe can give a convincing answer.

Simon Jenkins is a Guardian columnist