In 1907, the then Siamese government signed over the provinces of Battambang, Siem-reap, and Sisophon to French Cambodia. The treaty outlined the new land and sea borders between French Cambodia and Siam — now Cambodia and Thailand, respectively — whose interpretation became a point of contention between the two countries in 1972. Since then, relations between the Thai and Cambodian governments over the disputed maritime territories have been amicable, but Thai nationalist group pressure has stymied recent attempts to resolve the issue.
With the establishment of the Chinese-funded Ream port in Cambodia, there is increasing concern that the port could be converted to military use to strengthen Chinese naval power projection or be a competitor to Thailand’s Eastern Economic Corridor. The port and secrecy around its actual use are reigniting concerns first established in the 1907 treaty, and inflaming Thai worries about what the port means for its maritime territories, the stability of the Gulf of Thailand, and relations with China and the United States.
In full: https://responsiblestatecraft.org/cambodia-ream-naval-base-china/
