This is a broad statement to make, and is mostly a misrepresentation of Johnson's views. Don't say things you can't verify.
http://www.weeklystandard.com/blogs/yes-gary-johnson-endorsed-humanitarian-war_522029.html
Johnson has only endorsed the idea that the US has a moral right to, if it so chose, pursue war against a nation to stop genocide - but not to nation build or engage in a battle against a sizable enemy force. It was specifically referenced to the genocide happening in Africa, where military intervention would be a mostly non-violent endeavor due to their savage living conditions and low societal standards.
He has not endorsed anything beyond what a classical Objectivist (or Ayn Rand herself) would have endorsed - which most here, I would imagine, would find complimentary to their own views.