In light of the beginning of Attacks against Libya and the UN Security Council Resolution authorizing the establishment of a No-Fly Zone over part of Libya I thought it would be useful to have a post about Acts of War and historically what has been considered a legitimate reason to go to war. I will focus this post on the Westphalian System established in 1648 by the Peace of Westphalia ending the Thirty-Years War that also inaugurated the current system of Sovereign nation-states operative in the world today. The Westphalian System did not spring fully formed in 1648, mainly because it was focused on monarchical and dynastic states and not the modern nation-state. The concept of popular sovereignty would have been anathema to the leaders of 1648. That being said, the main tenets of international relations were established then, perhaps the most important of these ideas is the notion of sovereignty, specifically, internal sovereignty.
In a nutshell, internal sovereignty means that the duly constituted authority within a nation state is sovereign, or free to conduct its internal affair however it likes. This is regardless of whether its neighbors like its policies or not. Now in practice, this idea has often been ignored when convenient, but that just leads the post into a discussion of what an Act of War is. Because an Act of War is essentially anything that a sovereign state thinks infringes on its rights as a sovereign state.
The idea of and acceptance that recognized nations are sovereign is central to the discussion of whether action against Libya for putting down an internal rebellion is a breach of international law by those conducting the intervention i.e. the no-fly zone. If you look at traditional international law, as it has existed since 1648, the imposition of a no-fly zone on Libya absent a Libyan attack or other Act of War to another state, then the international community is violating Libyan sovereignty by imposing a no-fly zone.
The fly in the ointment is the UN, specifically, the UN Charter. This is not the place for a history of the UN, that properly belongs in another post entirely. Perhaps I will write one because the UN was founded as a direct response to World War II. For this post, suffice it to say that the whole purpose of the UN as originally envisaged is the maintenance of international peace. It is right there in the preamble to the UN Charter,

“WE THE PEOPLES OF THE UNITED NATIONS DETERMINED to save succeeding generations from the scourge of war, which twice in our lifetime has brought untold sorrow to mankind.”

The UN changed the international system and arrangements between and among states. It did not invalidate traditional international law and relations. Instead, it added another layer to international relations. The UN Security Council has arrogated to itself the right and authority to interfere in the internal affairs of both member and non-member states to keep the peace. Peace is very loosely defined in the charter and therefore means whatever the Security Council says it means. Now of course, the UN does not have its own military so it relies on the militaries of member states to enforce Security Resolutions. That is what we are seeing in Libya right now.
The question concerning Libya is why is NATO intervening there but not in Bahrain and Yemen where essentially the same thing is happening? I would submit that the intervention in Libya is because Libya is a pariah state while Bahrain and Yemen are western allies. Indeed, the US Fifth Fleet is headquartered in Bahrain. The current intervention in Libya is being conducted under the auspices of Chapter VII, Articles 39 & 42 of the UN Charter, which read: Article 39

“The Security Council shall determine the existence of any threat to the peace, breach of the peace, or act of aggression and shall make recommendations, or decide what measures shall be taken in accordance with Articles 41 and 42, to maintain or restore international peace and security.”

& Article 42:

“Should the Security Council consider that measures provided for in Article 41 would be inadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations.”

In closing, it is my opinion that Libya has been singled out, not because the legitimate government is engaged in putting down an insurrection, but because the Libyan regime is distasteful to Western governments and the current revolt is seen as a golden opportunity to get rid of Qaddafi. It remains to be seen whether they have made a smart decision or not. History shows that no-fly zones are fairly impotent. Saddam Hussein had very little trouble putting down two insurrections at once in the early 1990s despite the presence of a no-fly zone. So far, it seems despite initial reverses Qaddafi is well on the way to crushing the Libyan rebels as well.

Useful Links:
Benno Teschke, “Theorizing the Westphalian System of States: International Relations from Absolutism to Capitalism”, European Journal of International Relations, , Vol. 8, no. 1: 5–48 (2002)

War, Peace, and Internal Sovereignty-Unpublished, Draft Conference Paper. However, it is still available on the web and well worth reading.
The Crimes of War Project. I don’t necessarily agree with some. Or even most of their positions but this site and its attendant glossary/dictionary is a good place to start individual research into current International Law on the Use of Force between nations.
UN Security Council resolution 1970 (2011) establishing the Libyan no-fly zone

United Nations Charter

Geneva Conventions of 1949 and additional Protocols, Full-Text

Hague Conventions of 1907 on the Laws of War