At Lancaster House, a 19th century mansion adjacent to Buckingham Palace, it felt like a moment of truth for Europe, as the continent’s biggest powers gathered to try to rescue something from a crumbling postwar order.

The freeze in American aid poses a severe strategic problem for Ukraine, which has become reliant on military and economic support from its Western partners as it tries to repel Russia's three-year-old invasion and stave off President Vladimir Putin's push for a peace deal beneficial to Moscow.