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.
政府智囊团的联邦安全政策学院的Karl-Heinz Kamp表示，由普京造成的不同威胁，以及特朗普提出的 “如何震慑，震慑谁，又靠什么来震慑” 的问题，即使德国拥有了核弹并非最佳的解决方案。
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.