Libertarianism is a broad political philosophy which holds that every person is the owner of his life and should be free to do whatever he wishes with his person or property, as long as he allows others the same liberty. The foundation of this philosophy varies widely among libertarians, with the consequence of widely varying proposed implementations of libertarianism. For example, there are anarchist and democratic libertarians, pro and anti-abortionists, pacifists and hawks, and those who support or oppose intellectual property rights. Most Objectivists believe that the lack of any philosophical foundation leads to conflicting and irrational directions for the libertarian movement, making it worse than useless as a means to advance liberty.
Objectivists are commonly described as "libertarians" by the public, including most libertarians. This highlights the difficulty in defining a movement described by an abstract statement without a single philosophical basis. In reality, Objectivists disagree with the libertarian movement on a number of major issues, such as the need for government, the need for a selfish foreign policy, intellectual property, and other issues. Ayn Rand identified herself politically as a "radical for capitalism."
A common critique of libertarianism is that it attempts to merge the capitalism with pacifism. It is an unstable system which is closer to anarchy than conventional capitalism because it applies skepticism to all international relations. For instance, only retaliatory force is considered appropriate, not proactive measures to prevent destruction. Under libertarianism, you do not have a right to attack a person who is preparing to attack you, even if you wouldn't be able to attack them once the threat becomes "proven". Of course, because the libertarian is a skeptic, very little will convince them that there was a threat short of actual initiation of force. For an analogy of libertarianism in simple terms, the right to respond comes not when a person pulls their arm back to punch, but when they are in the process of actually punching.
Objectivism differs from libertarianism in this respect. A person, according to Objectivism, can have confidence in their own judgemental capabilities and determine that someone is a threat without there being any imminent signs. Rather, a person can judge from a person's character, social character, productive character, and capacity to attack, not just immediately, but in the future. In a social context where the authority in question is reliable (both qualified and trustworthy) to respond, one should rely on the authority, since they have a rightful monopoly of force. However, international situations are no such social context, and so action must be taken by the nation being threatened and its allies against the threatening nation and its allies.