Chinese patrol ships sent to disputed islands arrived in the East China Sea early on Tuesday. The islands are known to the Japanese as Senkaku, and to the Chinese as Diaoyu.
Xinhua News, China's official media organ, said the ships, part of the China Marine Surveillance (CMS), are there to "assert the country's sovereignty."
The news outlet added, "The CMS has drafted an action plan for safeguarding the sovereignty and would take actions pending the development of the situation."
The CMS is a paramilitary force, which, the Associated Press notes, is often "lightly armed."
The ships appear to be a tactical response to Japan's recent $26.18 billion purchase of the islands from their private Japanese owner.
Japan's Foreign Minister, Koichiro Gemba, said the purchase was for the "peaceful and stable maintenance of the islands."
China called the transaction "completely illegal and invalid."
National egos are at stake, but it's really the potential energy resources the countries could extract that are on the line.
Back in July, GlobalPost spoke with Andrew Billo, a senior program officer with the Asia Society who specializes in Southeast Asia, about the dispute. He said no one really knows what kind of "energy potential" the region has, though it's likely significant.
More from GlobalPost: Why all the fuss in the South China Sea?
As is often the case, the dispute has escalated incrementally — and probably will for some time — while the rhetoric has risen meteorically.
Consider Beijing's recent militaristic response:
"The determination and the will of the Chinese government and military to safeguard their territorial integrity are firm," Chinese Defense Ministry spokesman Geng Yansheng said. "We are closely monitoring the development of the situation and reserve the right to take necessary measures."