Japan and South Korea have long-established themselves as fervent allies of the United States, but recent altercations between the two Pacific nations have shown that the friend of a friend can still be your enemy.

On Wednesday, South Korean officials accused a Japanese military patrol aircraft of executing a “threatening” low-altitude pass over one of their destroyers as it sailed through international waters. This incident and the ensuing diplomatic fallout is just the latest spat between the two nations following a December incident in which Japanese officials claimed a South Korean vessel locked onto a Japanese aircraft with its fire control radar. The Japanese plane was reportedly involved in rescue operations for a North Korean fishing boat at the time of the incident, but Seoul claimed the aircraft flew dangerously close to its ship. South Korean officials have also claimed that the use of radar was not meant as a provocation, but that it had been activated to assist in searching for the fishing boat.

Both nations, as a result, have demanded apologies from one another.

According to General Suh Wook, the chief director of operations for South Korea’s Joint Chiefs of Staff, Wednesday’s low flyover was the third time a Japanese patrol aircraft buzzed a South Korean Navy vessel. He went on to warn that his nation’s military will respond if provoked.

“The threatening low-altitude flight conducted today, despite our firm request to the Japanese government to ensure that (such flights) don’t recur, is a clear provocation against the naval vessel of a friendly country and makes it impossible for us not to question Japan’s intentions,” Suh said in a statement. “If such activity repeats again, our military will respond strongly based on our response rules.”

Tensions between the two nations may well be exacerbated by their joint histories. Japanese forces occupying the entire Korean peninsula between 1910 and 1945 have been described by many historians as brutal, including how many women were forced in sexual slavery as “comfort women.” Anger regarding these atrocities has not waned over time, with South Korean courts recently ordering Japanese companies to compensate victims of the era. Japan, angered by this ruling, cited the 1965 treaty that formally normalized relations between the two countries as legal precedent barring any such payment.

Many in the South Korean media have accused Japanese Prime Minister Shinzo Abe of allowing tensions to escalate as a part of a larger effort to rally support within the nation for his initiative to overhaul the Japanese military. Japan has maintained only a “self-defense” force since the conclusion of World War II, but provocations by nations like North Korea have prompted Abe to seek a shift that would allow for offensive military operations in instances when national security calls for them.