But now, some 30 years after his brief stint in charge, the fierce anti-European stance of Donald Trump's administration and its outreach to Russia is making the ex-Belgian foreign minister queasy with nerves.

"Their disdain is incredible," Claes, 86, told AFP in an interview at his home.

"I can't understand why they're being so anti-European and showing solidarity with the Russians at the expense of their long-standing allies. It makes me sick."

Claes's tenure at NATO lasted just 13 months -- making him the shortest serving secretary general -- and ended in disgrace when he had to resign over a corruption scandal.

But it was a critical period for the alliance as it searched for its purpose after the collapse of the Soviet Union, and launched its first military operation to intervene in the Balkans.

Now NATO appears to be facing another decisive moment, as Trump rocks the foundations of the security order that has underpinned the West for the past seven decades.

Claes says he is "waiting impatiently" for NATO's summit in The Hague in June for Trump to give a sign if the alliance will continue in its current form.

"We will know whether or not the United States intends to respect the commitments made since NATO's creation. If not, it will be very painful for us Europeans," he says.

As questions hang over Washington's commitment, Claes insists that Europe needs to work on having "greater autonomy" in decisions taken at the alliance.

Back in his time, there were only 16 member countries in NATO -- but now it has 32 and finding unanimity has often become trickier.

Claes recognises that Europe is often divided as some countries such as Hungary "prefer dialogue with Moscow".

"How can we organise confidential decisions on armaments with partners like Hungary, Slovakia and even Turkey?" he says.

- Putin's ambitions? -

Russia has pitched its invasion of Ukraine as a reaction in part to NATO's moves towards its borders -- a narrative rejected by the West as a flimsy justification for President Vladimir Putin's expansionism.

Claes says that in his opinion it was US president George Bush who miscalculated in 2008 by pushing the alliance to say ex-Soviet Ukraine and Georgia would one day become members.

"He caught the French and Germans short," Claes said of Bush. "Putin thought that we were getting too close to his borders."

Shortly afterwards Russia launched a lightning war in Georgia -- setting the stage for the tensions to come.

Another error came in 2013 when Bush's successor Barack Obama backed down at the last minute on striking Syria after Bashar al-Assad's use of chemical weapons.

"Obama made a mistake, he gave Putin a signal of weakness. That's when the Kremlin said to itself 'it's time to get on with it'," he says.

For now Claes does not see an assault from Moscow against a NATO country as likely -- as the Russian army has been so weakened in Ukraine.

"But I can't hide my concerns about nuclear weapons," he added.

"There is an urgent need to restore dialogue between the major powers on the control of these weapons, and if possible on disarmament. The whole of humanity is at risk."