China’s escalating clashes with the Philippines over disputed waters in the South China Sea are forcing an uncomfortable question: At what point is the U-S obligated to help repel them with force? The Philippines is protected by a US defense treaty that is “ironclad,” as US officials reiterated this week. Attacks by the China Coast Guard at sea have steadily escalated from harassment to blockades. If this trajectory leads to deaths at sea, the Pentagon could be obligated to step in. The World’s Patrick Winn reports.