"Being a monopoly" is not necessary to be in violation of US antitrust law. Merely seeking to become one via restraining trade can put you on the wrong side of the law.

But this isn't the real issue. The real issue is that the US hasn't vigorously enforced its own antitrust law in over 20 years, because a policy of "monopolies are good" was adopted in the 80s and has only gotten stronger over the years. (The Microsoft antitrust case of the 1990s was an exception.)