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.