Since he started work as Nato secretary general almost two years ago, Mark Rutte has spent much of his time trying to keep the United States anchored to the world’s biggest military alliance, employing outright flattery to dissuade US President Donald Trump from acting on threats to abandon it. But the goalposts keep shifting, raising the stakes ahead of this week’s summit in Turkey. Initially, it was about money. Trump has long railed against Nato allies for spending too small a fraction of...