HONG KONG — A specter is haunting Washington, an unnerving vision of a Sino-Russian alliance wedded to an expansive symbiosis of trade and commerce across much of the Eurasian land mass — at the expense of the United States. And no wonder Washington is anxious. That alliance is already a done deal in a variety of ways: through the BRICS group of emerging powers (Brazil, Russia, India, China, and South Africa); at the Shanghai Cooperation Organization, the Asian counterweight to NATO; inside the G20; and via the 120-member-nationNon-Aligned Movement (NAM). Trade and commerce are just part of the future bargain. Synergies in the development of new military technologies beckon as well. After Russia’s Star Wars-style, ultra-sophisticated S-500 air defense anti-missile system comes online in 2018, Beijing is sure to want a version of it. Meanwhile, Russia is about to sell dozens of state-of-the-art Sukhoi Su-35 jet fighters to the Chinese as Beijing and Moscow move to seal an aviation-industrial partnership.