WASHINGTON, July 3 -- When federal security agencies began coordinating protection for the 2026 FIFA World Cup, the largest sporting event ever staged on North American soil, they funneled sensitive operational intelligence through a single unclassified network designed to let dozens of agencies share information without sharing it with the public. That network was breached.

The Department of Homeland Security disclosed Wednesday that its Homeland Security Information Network, known as HSIN, was compromised sometime between late May and early June. The disclosure came roughly a month after the breach was contained, and weeks after the World Cup had already begun. The intrusion, as TechCrunch reported Wednesday, targeted a platform that D...