Buried in a long US indictment accusing China of a global cyberespionage campaign was a curious detail: among the governments targeted by Chinese hackers was Cambodia, one of Beijing’s most loyal Asian allies. The target of the hack, which two sources with knowledge of the indictment said was Cambodia’s foreign ministry, was also revealing: discussions between China and Cambodia over the use of the Mekong River, a new battleground for US and Chinese influence in Southeast Asia. Four Chinese nationals – three security officials and a contract hacker – have been charged for attacks aimed at dozens of companies, universities...