There are always questions after any attack as to why it was not stopped or detected. But the Moscow attack raises particularly difficult issues for Vladimir Putin at a time of international tension and mistrust. And much of that comes because of a warning from Washington. The 7 March warning from the US to its own citizens was unusually specific. It talked of reports that "extremists" had "imminent plans to target large gatherings in Moscow" and specifically mentioned concerts. It advised Americans in the city to avoid large gatherings over the coming 48 hours. The timing may not quite match,...