Why didn't the USA try to stop China 40 years ago？

Why didn't the USA try to stop China's economic rise 40 years ago rather than now?

The USA tried to stop China’s economic rising 70 years ago. Since 1949, the USA sanctioned and blocked China, staged the Seventh Fleet in Taiwan, and waged the Korean War and Vietnam War following the Domino Effect doctrine of foreign policy after the Loss of China. You see, the USA is actually smarter than the questioner. The USA is 30 years ahead of the question.

The US lost the Vietnam War terribly. For one thing, the war cost the USA so much that the USA breached the Bretton Woods agreement and refuse to convert the dollar into gold. It means the Soviet was going to win the Cold War as the financial system, the Bretton Woods System, crashed. Second, the Soviet was ahead of the USA in Space Race. Soviet sent an astronaut into space first. Third, the US was still in the quagmire of the Vietnam War.

So, in order to maintain the position of the hegemon, all the USA has to do is sabotage number two. In the 1970s the USA himself was still number two, and the Soviet was number one.

In order to become number one, Nixon went to China asking for help. The USA needs help from Mao to get out of the quagmire of the Vietnam War. The USA realized 40 years ago that stopping China from rising did not help the USA's national interest, but letting China rise did. From 1979 to 1989, there were ten years of honeymoon between China and the USA. You see, the USA cannot eat the pie and still have it.

Then why did not the USA stop China from rising after the crash of the Soviet? Because China was not number two 30 years ago. 30 years ago, Japan was number two. The USA was busy digesting the Plaza Accord to stop Japan from rising.

After the Plaza Accord, the USA succeeded to stop the rising of Japan. Then why did not the US stop China from rising 20 years ago? Because 20 years ago, China was not number two yet. The EU was number two. So the USA started the Kosova war, an attack on Euro. Then the USA started the Iraq War, an attack on the Euro again to prevent the Euro as a currency to settle international trade of petroleum. The USA weaken the EU further by letting the EU expand into South Europe and East Europe, putting more hassles into EU politics. And had an orange color revolution to further weaken the EU. Now the EU is really dysfunctional, with Brexit and the refugee crisis.

Then why not the USA stop China from rising ten years ago? It did. Because 10 years ago, China became the number two. 10 years ago, Obama initiated Asia-Pacific rebalance, initiated TTP, had Japan dispute Diaoyu Island with China, had the Philippines and Vietnam dispute the South China Sea with China, etc. The USA has been stopping China from rising since 10 years ago. Trump did it only more shamelessly.