The UN's top court ruled Monday that the area around a flashpoint ancient temple on the Thai border belongs to Cambodia and that Thai security forces should leave.
The International Court of Justice interpreted a 1962 ruling saying that "Cambodia had sovereignty over the whole territory of the promontory of Preah Vihear," Judge Peter Tomka said.
"In consequence Thailand was under an obligation to withdraw from that territory Thai military or police forces or other guards or keepers who were stationed there," Tomka said.
At least 28 people have been killed in outbreaks of violence since 2011 over the ownership of the patch of border land next to the 900-year-old Preah Vihear temple.
Ahead of the ruling by The Hague-based ICJ, there were fears that the decision would revive nationalist tensions and spark renewed clashes.
Tens of thousands of people were displaced in the 2011 fighting, leading Cambodia to ask the ICJ for an interpretation of an original 1962 ruling.
Thailand does not dispute Cambodia's ownership of the temple, a UNESCO World Heritage site, but both sides laid claim to the adjacent 4.6-square-kilometre (1.8-square-mile) piece of land.
Leaders of the two countries called for calm ahead the ruling by 17 international judges. The court decision is binding and cannot be appealed.