Coinciding with the last day of US President Barack Obama’s Asia tour, Vladimir Putin Tuesday gave the green light to sell China Russia’s newest S-400 air defense guided missile system. Moscow says this will give Beijing an edge in the airspace over the Taiwan Strait and the Diaoyutai islands disputed between China and Japan. China voiced strong opposition to Obama’s pledge a week ago in Tokyo to come to Japan’s defense in any conflict with China over the islands.