Question

Around the time of the Cold War, why was China becoming a communist country such a scary thing for the United States?

Answer 1

Probably yes. China is an enormous country with some v clever people in it (most countries have clever people, china may have more 'cos it's a big pop). I don't know if C is a nuclear power ... but given that N Korea seems to be it's V probable that C is too. And being a communist country meant, to the US, possibly getting into the space race and the nuclear arms race, the one result of which was actually seen in about 1945 when US president Truman gave the go ahead for dropping nuclear/atom bombs on Hiroshima and Nagasaki in Japan, and thereby, among other things, ending Japan's war "effort" and signalling "VJ" day. At the cost of thousands of non combatants' lives some of whom were probably VAPORISED.
That's probably a tiny part of the overall truly historic set of events.
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Answer 2

Then there is the chinese DRACONIAN policy on birth control ...