The U.S. did nothing in the arrangement because their position wrt the CCP was one of engagement to balance against the USSR. It was the UK, which caved becaus

The U.S. did nothing in the arrangement because their position wrt the CCP was one of engagement to balance against the USSR. It was the UK, which caved because, as revealed in now-declassified democratic cables from 1958, Zhou Enlai flatly stated that the PLA would invade if HK was given independence. The UK quite simply could not defend HK. It had failed to defend it in WWII against Japan (it couldn't even defend Singapore) and would have no hope against the CCP, even without the Harbour Tunnel to the Kowloon Peninsula, which had already been operating for some time. Instead it opted for an agreement in the joint declaration and a hope that the CCP would lean more towards international norms as the Shanghai cadre assumed more control. Objectively a poor solution but, I have to say, probably the most rational. Of course, we all know what happened. Deng's best laid plans came to naught and another 'great leader' emerged.