As Russia was unleashing its deadliest attack on Ukraine’s capital in nearly a year, President Volodymyr Zelensky arrived in the Canadian Rocky Mountains early Tuesday hoping to meet with President Trump and secure more support for Ukraine from the Group of 7 industrialized nations. Instead, Mr. Trump left the global powers’ summit early, canceled his meeting with Mr. Zelensky, lamented Russia’s absence from the leaders’ get-together and rejected the idea of issuing a joint statement in support of Ukraine. A single, brief reference to Ukraine in a summary of the talks’ conclusions, released by the host, Prime Minister Mark Carney...