For years, international relief workers and the Israeli military have reported weapons caches occasionally found in schools operated by the United Nations Relief and Works Agency, the organization that for decades has provided schooling, healthcare and other assistance to Palestinian refugees in Gaza. They learned of underground tunnels beneath Unrwa facilities and the theft by Hamas of agency-provided fuel and aid. Some had run-ins with teachers over textbooks promoting the hatred of Jews and Israel. In 2014, part of the parking lot at the Unrwa headquarters in Gaza began sinking, likely from a Hamas tunnel dug beneath. “No one talked...