IT BEGAN in November, soon after the election of Donald Trump as America’s president. The publisher of the Frankfurter Allgemeine Zeitung, a conservative newspaper, opined in an editorial that it was time to contemplate “the altogether unthinkable for a German brain, the question of a nuclear deterrence capability, which could make up for doubts about American guarantees”. Roderich Kiesewetter, a foreign-policy expert in the Christian Democratic Union, the party of Chancellor Angela Merkel, chimed in that there should be no “thought taboos”. He and other politicians then went silent, apparently after a signal that the chancellor did not need this distraction in an election year. But in Germany’s think-tanks the debate kept raging.

Since 1945 West Germany and then the reunited country have relied on the American nuclear shield to deter aggression from Russia. A prominent thesis, outlined in 1984 by Josef Joffe, a journalist, holds that European integration was only possible because this external American power had “pacified” the age-old Franco-German conflicts. So West Germany, on its best behaviour after the war, signed the non-proliferation treaty in the 1960s; it reaffirmed the pledge in the treaty that led to reunification in 1990.

Suddenly, however, there is an American president who, though he said last week that he would “strongly support NATO”, has also called the alliance “obsolete” and suggested that his support might be conditional on allies meeting their commitments to spend more on defence. By the ghastly logic of mutual assured destruction (MAD), deterrence must be unconditional to be credible. Countries in eastern and central Europe are beginning to fret about their vulnerability to nuclear blackmail by Russia under Vladimir Putin.

Germany’s most obvious response would be to approach France and Britain, NATO’s other two nuclear powers, for a shared deterrent. But their arsenals are small. France, moreover, has so far been unwilling to cede any sovereignty over its nuclear arms and has always been sceptical about shared deterrence. Britain, as its prime minister, Theresa May, has already hinted, might make its nuclear shield a subject of negotiation during the upcoming Brexit talks.

To Maximilian Terhalle, a German professor currently teaching in Britain, this means that Germany, Poland or the Baltic countries could never fully rely on France or Britain retaliating against Russia for a strike against them. He concludes that Germany must think about getting its own nukes, perhaps in collaboration with neighbours. Even the leader of Poland’s governing party, Jaroslaw Kaczynski, a habitual Germanophobe, called in February for a European nuclear deterrent, presumably financed largely by Germany.

The different dangers posed by Mr Putin and Mr Trump have raised the question of “how to deter whom with what”, even though German nukes are not the best answer, says Karl-Heinz Kamp of the Federal Academy for Security Policy, a government think-tank. Mr Terhalle, for his part, thinks that even a debate about a German nuclear weapon could help—if it convinced Mr Trump to stop undermining the existing international order.