As he was rushing to attend a soccer match in Newcastle, England, Michael Gibbard stepped on a pile of sensitive military documents. At first, he thought the papers scattered along a street near the stadium were trash, dumped there from a nearby office building. But when he bent down and examined them closely, he was stunned by what he saw. Names and ranks of soldiers. Detailed base patrols. Drug test results. Codes to weapon armories. “I thought, bloody hell, this shouldn’t be here,” said Mr. Gibbard, a 41-year-old owner of a delivery service. Mr. Gibbard's accidental discovery this month of what appeared to be hundreds of military documents on an industrial street in Northern England has shocked a country known for zealously protecting state secrets. It also comes as the United States, Britain’s close military ally, has been facing its own crisis over the handling of sensitive national security information, after battle plans in Yemen were discussed in a group chat that included a journalist.