The world watched last week as the Republican Party in the United States conferred its nomination for President on a man of doubtful political pedigree and unpredictable intentions. The New York property tycoon has not been a conventional Republican and has not run a conventional campaign. That seems to be his appeal to enough American voters to secure the nomination. Whether it will appeal to four times as many voters at the election in November remains to be seen. Between now and then, everything Donald Trump says needs to be taken very seriously.

In an interview with the New York Times last week, he suggested that as President he would not necessarily stand by the security guarantees the US provides for members of the Nato Alliance. Asked whether he would extend that to the small Baltic States if Russia attacked them, he said he would make that decision only after reviewing whether those states "have fulfilled their obligations to us". There is only one obligation they owe the US under Nato and it is the same one that all alliance members, including the US, have accepted: to regard an attack on any one of them as an attack on them all.

Possibly Trump is not aware of that. He has not been well versed on the detail of many of the issues he has raised. But his precise meaning is less important than the instincts he reveals. No previous presidential candidate, especially from the Republican Party, would have left any doubt of his commitment to Nato partners. The slightest doubt where countries on the borders of Russia are concerned can only encourage the sort of adventurism Vladimir Putin has already displayed in Georgia, Crimea and the Eastern Ukraine, and put the Baltic States at greater risk.

Trump must be aware of that much. His prevarication on the Nato commitment appears deliberate and it is not an isolated remark; earlier this year he suggested the US role in the alliance should be scaled back and last week the Washington Post reported the Trump campaign had worked to soften the language of an American commitment to Ukraine, which is not in Nato but would like to be. Trump may just be ultra-cautious and keeping all his options open if he is in power next year, but there is reason to think he would bring a radical change in America's foreign engagements.

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He seems to regard foreign security commitments with much the same distaste he has for trade deals. He sees them as weighted against the US and thinks one way to "make America great again" is to withdraw from them if they do not serve a very narrow view of America's needs. He is playing to the isolationist instinct that has always been a popular force in American politics, as it is in most countries. Britain's vote to leave the European Union was grounded in the same impulse. Brexit and the Trump campaign are being widely interpreted as a retreat from globalisation.

The retreat is unlikely to succeed in economic terms. Trade and technology has permeated too deeply. But defence treaties are easily weakened, and can be undone with a careless word.