The United States accused China on Friday of failing to disclose a 2020 nuclear test, violating what a US official suggested was an understanding involving a 1996 nuclear test ban treaty that both countries signed but neither ratified. This came as Washington called this week for a broad new arms-control treaty to include China, Russia and the US after it declined to extend the New Strategic Arms Reduction Treaty (Start), the world’s last binding nuclear arms control agreement. Russia had...