1. The purist:
Liberty -- the right to do as one wishes as long as one does not harm others -- is an inherent right, something that humans are born with. Therefore, no one need prove than liberty is superior to statism or other totalitarian philosophies (e.g., Islamism). Liberty would reign in a world of statelessness, where all relations and transactions are consensual.
2. The realist:
The precise contours of liberty depend very much on agreement about harms, which are defined through politics (interpersonal and intergroup bargaining). Liberty, thus defined, is sustained by defending it politically and, as necessary, with force. The justification for liberty (or more rather than less of it) depends very much on evidence that it is superior to statism or other totalitarian philosophies.
Not all nations and peoples subscribe to the notion of liberty. Those who do must be prepared defend it against those who do not, even to the point of acting preemptively. Moreover, the government of a nation must be prepared to defend liberty over the objections of some of its citizens -- and by means that not all will applaud.
Liberty -- like economic and scientific achievements -- requires leadership as well as cooperation, it does not simply "happen." The ideal world of stateless consent is just that: an ideal. The real world is fraught with predators, persons of ill will (e.g., persons whose allegiance is to party rather than nation), and dupes. Predators succeed in crushing liberty where a nation's politics are dominated by dupes and persons of ill will.
Related: Consent of the Governed and the many posts linked therein, especially these:
Practical Libertarianism for Americans (links to a series; see especially The Origin and Essence of Rights)
The Meaning of Liberty (a series gathered in a single post)
Actionable Harm and the Role of the State
Varieties of Libertarianism
