A prominent Vietnamese historian who criticized his government for not doing more to challenge Beijing’s territorial claims in the South China Sea has been ousted from Vietnam’s ruling Communist Party over comments he made on Facebook.

The political purge of Tran Duc Anh Son, an expert on Vietnam’s claims in the South China Sea, is a rare window into how the party handles dissent among its rank-and-file members. It may also underline the sensitivities around Vietnam’s handling of its relationship with China, its largest trading partner and former imperial occupier.

Vietnam’s state-run news media reported last week that Dr. Son, who is in his early 50s and worked for years at a state-run research institute in the central city of Danang, was expelled for posting false information and violating a code that governs party members’ behavior.

“I knew this day would come,” Dr. Son said in an interview over a messaging service. He closed his Facebook account this week, saying he needed more time to work on book projects and transition to a new job as the director of a publishing house.