美国以国家安全为由打击中国企业. In this sentence, 以 means " use " and 由 means " reason" If 以 and 由 are used together, in fact, it is a sentence pattern. for example 以...为由, this means " take something as a reason. Specifically, in this sentence, 美国以国家安全为由打击中国企业， it means 国家安全 is the reason which is used by America to attack Chinese companies.

以 some reason 为由 do sth=用the reason做理由 do sth.

It is not suitable to use 用 in this sentence. Because as I said above, 以...为由 is a sentence pattern which is used together with reasons and in this sentence, 国家安全 is the reason. The word “ use" has the same meaning as 用 in Chinese. Specifically, we always say " use something" in English that is the same to Chinese. If you want to use 用, you must add the real thing you use behind 用. And in this sentence 国家安全 is a reason not a specific substance. I can give you an example, 我用右手写字---I use my right hand to write.

“以”This word has a lot of kinds usage, but the most common meaning is to use “以”is equal to“用”（use）Is a verb “由”is “理由”（season）Is noun