Sir Keir Starmer, Emmanuel Macron and Friedrich Merz have become adept at scrambling to deal with the latest bad news from Washington. Their meeting with Volodymyr Zelenskyy in Downing Street on Monday was so hastily arranged that Mr Macron needed to be back in Paris by late afternoon to meet Croatia’s prime minister, while Mr Merz was due on television for an end-of-year Q&A with the German public. But diplomatic improvisation alone cannot fully answer Donald Trump’s structural threat to European security. The US president and his emissaries are trying…