Israeli forces have taken control of much of northern Gaza—at least the parts that are above ground. Beneath the strip’s devastated urban landscape, Hamas still reigns.

TEL AVIV—Israeli forces have taken control of much of northern Gaza—at least the parts that are above ground. Beneath the strip’s devastated urban landscape, Hamas still reigns.

The war is entering a new phase, as the Israeli military takes its fight underground and into Gaza’s legendary subterranean tunnel network.