It was supposed to cap a week of whirlwind diplomacy advancing the prospect of peace in Ukraine.

But a summit of European leaders on Sunday has been overshadowed by the extraordinary scolding by U.S. President Donald Trump of Ukrainian President Volodymyr Zelenskyy at the White House on Friday for being ungrateful for U.S. support. The London meeting has now taken on greater importance in defending the war-torn ally and shoring up the continent's defenses.