If someone assaults you, you have the right to authorize government to punish those responsible in your behalf. Retaliatory force is justice. Let me say that again, Libertarians are those who oppose the initiation of force. Some libertarians are pacifists. They decline the use of any force.
Libertarianism is broad enough to encompass pacifists.
All oppose the initiation of force. Some libertarians are militant. Libertarianism is broad enough to encompass militants. 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.