In a remarkable show of persistence, North Korea on Wednesday fired two suspected powerful new Musudan mid-range missiles, U.S. and South Korean military officials said, but at least one of the launches apparently failed, Pyongyang’s fifth such reported flop since April.

Despite the repeated failures, the North’s determination in testing the Musudan worries Washington and its allies, Tokyo and Seoul, because the missile’s potential 3,500-kilometer (2,180-mile) range could target much of Asia and the Pacific, including U.S. military bases there.

Each new test — apparently linked to a command from North Korean leader Kim Jong Un — also likely provides valuable insights to the North’s scientists and military officials as they push toward their goal of a nuclear and missile program that can threaten the U.S. mainland. Pyongyang earlier this year conducted a nuclear test and launched a long-range rocket that outsiders say was a cover for a test of banned missile technology. […]