Legislators in the US state of Virginia approved a bill requiring school textbooks to include the Korean name for the sea between Japan and South Korea, in a major win for Seoul.
The two key US allies, battling renewed tensions dating from imperial Japan, fought a proxy battle over the textbook change, with their respective ambassadors repeatedly visiting Richmond, the capital of the southern state.
Korean-American activists gained the upper hand in their effort to address historical grievances with Japan at the local level in the United States.
The Virginia House of Delegates voted 81-15 on Thursday to approve the measure requiring "that all textbooks approved by the Board of Education [after July 1, 2014] when referring to the Sea of Japan, shall note that it is also referred to as the East Sea".
The bill, which has already cleared the Senate, now awaits the signature of Democratic Governor Terry McAuliffe, who is expected to sign it into law.
There was no immediate response from Japan, but the South Korean government, which had enthusiastically supported the bill, welcomed its passage.
"This is the result of efforts by our compatriots in the US to make the name of the East Sea used more widely," said foreign ministry spokesman Cho Tai-young.
Japan, which opposed the textbook change and hired a team of lobbyists to defend its position, had stressed that Sea of Japan was the only name for the body of water that was recognised by the United Nations and the International Hydrographic Organisation.
Sea of Japan was already in common usage in the world in the early 19th century. But Koreans say the name is a legacy of Japanese colonialism, and prefer to use East Sea.