If someone assaults you, you have the right to authorize government to punish those responsible in your behalf. Retaliatory force is justice. Some people have suggested a fourth category of preemptive force but most examples of preemptive force, upon analysis, can be placed in one of the other three categories. Libertarians are, by definition, those who oppose the initiation of force. SOME libertarians are also pacifists and decline the use of any force.
Libertarianism is broad enough to encompass pacifists.
All oppose the initiation of force. Libertarianism is broad enough to encompass militants. The common factor is opposition to the initiation of force. Freedom is the absence of the initiation of force. The robber and the bully have initiated force and the condition of freedom doesn't exist unless there is an absence of the initiation of force.