In the fall of 2020, the National Security Agency made an alarming discovery: Chinese military hackers had compromised classified defense networks of the United States’ most important strategic ally in East Asia. Cyberspies from the People’s Liberation Army had wormed their way into Japan’s most sensitive computer systems.

Sign up for Fact Checker, our weekly review of what’s true, false or in-between in politics. The hackers had deep, persistent access and appeared to be after anything they could get their hands on — plans, capabilities, assessments of military shortcomings, according to three former senior U.S. officials, who were among a dozen current and former U.S. and Japanese officials interviewed, who spoke on the condition of anonymity because of the matter’s sensitivity.

“It was bad — shockingly bad,” recalled one former U.S. military official, who was briefed on the event, which has not been previously reported.