President Donald Trump may have expected a Russian shudder of fear to greet his threat of “very severe tariffs” on the country if it didn’t agree a ceasefire in Ukraine.

His spectacular pivot involves ambiguity, theatre and cynicism. Yet it creates a pathway for Ukraine and its supporters to stabilise a weakening position. They must take it.

“The Patriot systems … will certainly make a difference to the defense of Ukrainian cities, particularly Kyiv,” Rajan Menon, a professor emeritus of political science at the City College of New York,...