A Japanese coast guard vessel, left, sails along with a Chinese surveillance ship in April 2013 near the disputed islands called Senkaku in Japan and Diaoyu in China in the East China Sea. (Kyodo News via AP)

As tensions over a disputed group of islands in the East China Sea, known as Senkaku Islands in Japan and the Diaoyu Islands in China, refuse to go away, there is an important set of words that U.S. readers should probably know.

The passage and the 103 words it contains are in Article 5 of the U.S.-Japan Treaty of Mutual Cooperation and Security. They read:

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. Any such armed attack and all measures taken as a result thereof shall be immediately reported to the Security Council of the United Nations in accordance with the provisions of Article 51 of the Charter. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.

These words are very important as they essentially tie the security of the two nations together. Speaking in Tokyo on Thursday, President Obama confirmed that yes, Article 5 of the treaty would extend to the contested islands in the East China Sea. "The treaty between the United States and Japan preceded my birth," Obama explained carefully, "so obviously, this isn't a ‘red line’ that I’m drawing."

It was his second confirmation in recent days. When asked Wednesday to officially confirm that Article 5 includes the East China Sea islands by Japanese newspaper Yomiuri Shimbun, Obama was unequivocal: "The policy of the United States is clear — the Senkaku Islands are administered by Japan and therefore fall within the scope of Article 5 of the U.S.-Japan Treaty of Mutual Cooperation and Security. And we oppose any unilateral attempts to undermine Japan’s administration of these islands.”

The new comments from Obama do not represent a change in policy (in 2012, then-U.S. Defense Secretary Leon Panetta said something similar), but they do appear to be the first time a U.S. president has confirmed that Article 5 would apply to the Senkaku/Diaoyu Islands. It's not necessarily surprising that Obama would make such comments on his Asia trip, which appears designed to reassure many Asian nations worried about the rise of China and its strength, both economically and militarily.

The disputed islands (Laris Karklis / The Washington Post) The disputed islands (Laris Karklis / The Washington Post)

However, given the rising tensions over the East China Sea islands, the comments are significant. While the islands have been claimed by Japan since 1895, China has increasingly argued that its own historical claims to the islands go further back and are thus more valid. In 2012, China released a number of maps that show the islands as its territory, while last year, the China’s Foreign Ministry released a statement saying that the island's were a "core interest" (i.e., worth going to war over). In Japan, nationalist politicians have also made the islands a rallying cry – in 2012, the Japanese government bought the island's from their private owner, in what appeared to be a bid to stop Shintaro Ishihara, a populist, nationalist leader, from buying them himself. There are significant concerns that a skirmish in the region could easily turn into a broader regional conflict if things went wrong.

The U.S.-Japan Security Pact is said to be one of the most successful pacts between two major powers ever: In 2010, George R. Packard wrote in Foreign Affairs that it had lasted longer than any major treaty since the 1648 Peace of Westphalia. It's rarely been put to the test like this, however.

Thankfully, both Obama's language and the language of Article 5 itself (which gives a big role to the United Nations in solving any dispute) are relatively cautious. And it's not entirely clear what exactly would be required if a conflict broke out. "Article 5 is deliberately vague regarding exactly what the United States would do in response to an attack on Japan," M. Taylor Fravel, an Associate Professor of Political Science Massachusetts Institute of Technology, told me in e-mail. "That would depend on the nature of the attack and the military requirements of a response. For example, in a limited conflict over the Senkakus, it is plausible that the US would provide reconnaissance and logistics support for Japanese forces that were defending or retaking the islands. Of course, the US would also provide non-miltiary support at the same time."

Still, China's reaction to Obama's statements appears to show how seriously they are taking it. "The so-called security alliance between the U.S. and Japan is a bilateral arrangement made during the cold war period, and it should not be used to damage China's sovereignty and legitimate interest," foreign ministry spokesman Qin Gang said in a statement, according to the South China Morning Post. "We resolutely oppose applying the Diaoyu Islands to the Japan-U.S. security treaty."