We had agreements to defend the Ukraine too and we saw how little those meant.
Check it out:

President Barack Obama—for the first time as an incumbent U.S. president—clearly stated the Senkaku Islands in Okinawa Prefecture are subject to Article 5 of the Japan-U.S. Security Treaty, in a written reply to questions submitted by The Yomiuri Shimbun.

“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 U.S leader stated ahead of his visit to Japan starting Wednesday.

Article 5 stipulates U.S. defense obligations to Japan, which apply to territories under the administration of Japan. Obama’s comment therefore means the United States will defend Japan in the event of a Chinese incursion on the islets, over which China also claims sovereignty.

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