The Western powers now bombing Libyan are protecting their easy access to Libyan, Saudi and Gulf oil and their overall control of the Arab World, which they have dominated since Britain and France carved up the Turkish-dominated Ottoman Empire after World War One (1914-18).
The rise of Arab Nationalism after World War Two threatened Western control, but with the help of Israel (formed by Jewish settlers in what had been Palestine in 1948), the US was able to reassert Western control.
Now the structure of imperial domination has taken a huge hit from the uprisings, particularly the fall of Mubarak in Egypt. The US and its allies know they must do something to ensure that whatever new order emerges it will still be under their control.
That easy bit, for them, is backing the Saudi / Kuwaiti / UAE invasion of Bahrain, to ensure the revolution is crushed there and does not spread. The kings of Saudi Arabia and of the smaller Gulf states have been important US allies for decades, providing cheap oil, military bases and political support within the Arab world. For any of these regimens to fall to democracy would be a huge set back for the US.
The hard decision was deciding what to do in Libya. When he first came to power 1969 Gaddafi was inspired by the Arab nationalism and Chinese Maoism. But the nationalists had already been defeated by Israel in the Six Day War of 1967, after which it occupied Gaza and the West Bank, as well as large parts of Syria and Egypt.
Like Syria, Libya remained independent from the US, but anti-imperialist and socialist rhetoric masked brutal dictatorships. Over the last ten years Gaddafi has made his peace with the West, but he is not dependent on them, and therefore not a reliable ally.
Waging war, nominally in support of, and at the request of, the rebels is a golden opportunity for the West to relegitimise military attacks in the Middle East after the disasters of Afghanistan and Iraq. It also offers the possibility of a new government dependant on Western support controlling either part or all of Libya and its oil. Most importantly, it allows the West to assert some control in this vital part of the world.