The most informative development in the Korean ship-sinking case this week is the silence of China on the matter, something South Korea’s press has addressed in pointed fashion. The Chinese announced on the 19th, moreover, that their ambassador would send a deputy to Thursday’s high-level diplomatic briefing from the South Korean government rather than attending it himself. Editorial staffs in Seoul interpret this as de facto support for North Korea’s position in the confrontation. They have reason to.

In the month before the sinking of South Korean navy corvette Cheonan (on March 26), North Korea extended to 2028 China’s lease on the eastern port of Rajin, which sits on the Sea of Japan. China is modernizing the port extensively for commercial use; Japan and South Korea have the obvious concern that China might begin sending warships there as well. In the month after Cheonan’s sinking, North Korea switched partners in its flagship tourism venture from South Korea to China. China’s tourists lost no time in taking advantage of that opportunity: the first tourist train from China entered North Korea on April 24. Tourism is a latecomer to the burgeoning trade between China and North Korea, which reportedly hit an all-time high in the first two months of 2010.

China’s proprietary relations with North Korea face an aggressive rival in Russia, which obtained a new 50-year lease on the Rajin port in March and plans to connect the port to its eastern railway system. Maintaining China’s position as Pyongyang’s principal patron is high on Beijing’s priority list, which explains why the Chinese welcomed a rare visit from Kim Jong-Il in early May and allowed North Korea to capitalize on that trip with its first-ever national display at the World Expo in Shanghai. (The chirpy cluelessness of MSNBC’s coverage here is priceless.) Neither the Cheonan incident nor reports in April that Pyongyang is planning a third nuclear test threw a damper on the fraternal amity blossoming in Northeast Asia.

The sense among China’s leaders that they have the latitude to display their true intentions in Korea has grown markedly in the last year. It was never accurate to perceive China as a like-minded ally of the U.S. in the Six-Party talks but, as late as April 2009, Beijing was still making a show of acting from common interests. That it no longer does can be attributed largely to the passivity and incoherence of the Obama administration. The administration’s only serious diplomatic response during the tense period after Cheonan’s sinking was to offer food aid to North Korea if it would rejoin the Six-Party talks.

But China has other examples to draw from as well, such as Obama’s unrealistic handling of Iran. The parallels between the Iran and Korea situations include, of course, multiple rounds of toothless international sanctions and U.S. bluster unsupported by any effective action. In the case of the Cheonan sinking, they also include a very specific analogue: the North Korean naval weapons involved. The analytical team’s finding is that North Korea used a Yono-class “midget” submarine to launch a former-Soviet-style 21-inch torpedo — the world’s most common type — at the South Korean corvette. Iran has produced seven Yono-design hulls as its Ghadir class since 2007, has fitted them to launch 21-inch torpedoes, and began adding them to the fleet in 2009. Iran, like North Korea, has been under UN sanctions throughout that period.