WASHINGTON—The classified documents that investigators say were leaked by a junior member of the Massachusetts Air National Guard indicate the extent to which U.S. spy agencies rely on clandestinely intercepted communications to keep tabs on their adversaries and allies alike. In vivid examples, the documents track foreign governments’ military movements, diplomatic efforts and clandestine weapons sales, as well as debates in friendly capitals and more. The powerful eavesdropping program that enables some of that intelligence gathering is due to lapse at the end of the year. To persuade Congress to renew the program, Biden administration officials had been debating whether...