Twitter is blowing up over this as I’m writing. The best thing I can do to get you up to speed is send you to this nifty Foreign Policy precis of the standoff. Nutshell version: Japan says the islands, which it calls the Senkaku, belong to it. (In fact, the Japanese government formally purchased them from their private owners just two days ago, partly to prevent them from being bought by Japanese nationalists and developed to spite China.) China says that the islands, which it calls the Diaoyu, actually belong to it per post-WWII agreements. The U.S. is obliged by treaty to defend Japan if it’s attacked; on the other hand, the agreements China’s citing to stake its own claim to the islands were authored by the U.S. Time for the Foreign-Policy President and Hillary to huddle. Quickly.

Two eternally rival nations, one a rising power and the other fading, locked in a territorial dispute that stems in part from the last time they went to war with each other. What could go wrong?

In 1971, the U.S. and Japan signed a treaty reverting Okinawa and the surrounding islands back to Japanese control, which stated “the United States of America relinquishes in favour of Japan all rights and interests under Article III of the Treaty of Peace with Japan signed at the City of San Francisco.”

China points to its own agreement with the United States, the 1943 Cairo statement issued by Winston Churchill, Franklin Roosevelt and Chiang Kai-shek. The three leaders agreed that following the war, “Japan shall be stripped of all the islands in the Pacific which she has seized or occupied since the beginning of the first World War in 1914, and that all the territories Japan has stolen from the Chinese, such as Manchuria, Formosa, [Taiwan] and The Pescadores, shall be restored to the Republic of China.” If you accept, as Beijing does, that Diaoyu was part of Chinese-ruled Taiwan prior to 1895, then that would indicate that it would be returned to China along with the other seized islands. (It would also mean that the U.S. military was bombing Chinese territory throughout the ’50s and ’60s, but that’s another issue.)

Despite the history, the U.S. State Department has stated that “the US does not take a position on the question of the ultimate sovereignty of the Senkaku Islands.” But further complicating matters is the 1960 Treaty of Mutual Cooperation and Security between the United States and Japan. According to Article 5 of the treaty, “Each Party recognizes that an armed attack against either Party in the territories under the administration of Japan would be dangerous to its own peace and safety and declares that it would act to meet the common danger in accordance with its constitutional provisions and processes.”

A State Department official told a Japanese news agency in July that the islands do fall under Article 5, so either the U.S. will be drawn in here on Japan’s side or else it’ll break the treaty to avoid a confrontation with China. (What that would mean for further Chinese aggression in the East China Sea, including Taiwan, who knows?) Anyway, the news tonight from Reuters:

Two Chinese patrol ships entered Japanese territorial waters near disputed islets claimed by both Beijing and Tokyo on Friday, Japan’s Coast Guard said, marking the latest incident in a long-running dispute between China and Japan.

The Chinese ships have not followed the coast guard’s order to go out of the territorial waters, a Japan coast guard official said.

According to Chinese media, they’re surveillance ships. Russia Today — which is a Kremlin-backed news outfit, for what it’s worth — says there are actually eight Chinese ships in the area, four inside the disputed zone and four outside, and that Japan has set up a “crisis center” and “urgently” summoned the Chinese ambassador. There’s another report floating around that, as of last night, three more Chinese surveillance ships were spotted near the Scarborough Shoal, another disputed territory — this one between China, Taiwan, and the Phillippines. Looks like China’s sending a message to all of its regional rivals at once. The Phillippines is itself ready to send ships to the shoal; no word yet on whether the Chinese ships are still there.

I’m seeing next to nothing on this so far in U.S. media, but Raymond Pritchett’s Twitter feed is chock full of new info. (Most of the above links are harvested from him.) Stick with his feed for updates. One interesting wrinkle: Apparently there’s a typhoon headed for the islands tonight, which might be the pretext used by both sides to pull out and save face/declare victory. Panetta’s scheduled to travel to the region “soon.” No doubt it’ll be sooner than he’d planned.