Leon Panetta said the president’s longstanding promise to withdraw soldiers is causing such concern in European capitals that plans are being considered for what would happen in the aftermath.
It comes as countries in the east of the continent – where many US troops are based – are increasingly worried about potential aggression from Russia.
“They are scared to death,” Mr Panetta, who served from 2011-13, told the McClatchy news site.
“They are worried about a very unpredictable president of the United States. They are increasingly worried he is going to do things not based on what’s in the best interest… but based solely on his vision of ‘America First’.”
Allies are especially concerned Mr Trump may make an unannounced promise to reduce troops – or American participation in Nato exercises – when he meets Russian president Vladimir Putin in Helsinki on 16 July.
When he held one-to-one discussions with North Korean dictator Kim Jong-un last month, he promised to stop joint military drills with South Korea – completely blindsiding leaders there.
Some 60,000 US troops are stationed in Europe, including 35,000 in Germany, 12,000 in Italy, 8,500 in the UK and 3,300 in Spain. They also form large swathes of the 8,000 Nato troops in countries on Russia’s border, including Poland and the Baltic states.
But the Pentagon is already reviewing the possible impact of reducing these numbers – with specific focus on Germany – according to a previous Washington Post report.
Erik Brattberg, director of the Carnegie Endowment’s Europe programme, said: “At a time when the transatlantic relationship between Europe and the US is under a lot of pressure over disagreements on Iran and trade, Nato is really at the core of this relationship and will Trump – by basically criticising the Europeans and conditioning American support – bring more disunity within the alliance.
“It would weaken the alliance and provide new opportunities for countries like Russia to take advantage of that.”
Mr Trump has repeatedly threatened to punish allies if they do not spend enough on defence, hinting the US may not protect those countries which do not increase their share.
In June, he sent letters to several Nato members complaining they were not abiding by a 2014 commitment to spend 2 per cent of their gross domestic product on national defence – which, said Kay Bailey Hutchison, US ambassador to Nato, resulted in all 29 members increasing their contributions.
The Independent
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