Can you please explain why you think that it would be opportunist to have relations with countries that offer some bit of reinsurance concerning the immediate threat of another country? In the case of China, the immediate threat was not the United States, it was the Soviet Union (check out http://www.telegraph.co.uk/news/worldnews/asia/china/7720461/USSR-planned-nuclear-attack-on-China-in-1969.html and don't forget that the USSR supported the Vietnamese attempt to form an Indochina Federation because having Soviets troops to the south of China and to the north of China would encircle them.) This is not to say that the United States was not the primary enemy, it was. The immediate enemy and the primary enemy are two different things, but within the context of the Chinese viewpoint, would it not make sense to have better relations with countries which could be used as a bulwark against Soviet aggression, without supporting the overall structure of the government in those countries?