Divisions within Germany’s Social Democrats over rearmament and relations with Russia are set to come to a head at a congress starting Friday, as party leader and finance minister Lars Klingbeil faces a backlash from the party’s old guard.

One critic is the eldest son of former SPD chancellor Willy Brandt, whose Ostpolitik of rapprochement with the Soviet Union at the height of the cold war still looms large over the party.

Peter Brandt, who says he never saw completely eye to eye with his father, said he signed the manifesto because he thinks the Russian threat is overblown.

“I do not share the idea that Russia is going to attack Nato,” he said. “The Russian army has shown weaknesses in the Ukraine war.”

He added that Nato “is now conventionally superior to the Russian army. Even without the Americans”. He also described Nato’s newly adopted goal of spending 5 per cent of GDP on defence as “irrational”.

“The rational approach should be: you do a threat analysis first.”