Walter Russell Mead writes in today's Journal on Why We're in the Gulf (sub req):
For the past few centuries, a global economic and political system has been slowly taking shape under first British and then American leadership. As a vital element of that system, the leading global power -- with help from allies and other parties -- maintains the security of world trade over the seas and air while also ensuring that international economic transactions take place in an orderly way. Thanks to the American umbrella, Germany, Japan, China, Korea and India do not need to maintain the military strength to project forces into the Middle East to defend their access to energy. Nor must each country's navy protect the supertankers carrying oil and liquefied national gas (LNG).
For this system to work, the Americans must prevent any power from dominating the Persian Gulf while retaining the ability to protect the safe passage of ships through its waters. The Soviets had to be kept out during the Cold War, and the security and independence of the oil sheikdoms had to be protected from ambitious Arab leaders like Egypt's Gamal Abdel Nasser and Iraq's Saddam Hussein. During the Cold War Americans forged alliances with Turkey, Israel and (until 1979) Iran, three non-Arab states that had their own reasons for opposing both the Soviets and any pan-Arab state.
When the fall of the shah of Iran turned a key regional ally into an implacable foe, the U.S. responded by tightening its relations with both Israel and Turkey -- while developing a deeper relationship with Egypt, which had given up on Nasser's goal of unifying all the Arabs under its flag.
Today the U.S. is building a coalition against Iran's drive for power in the Gulf. Israel, a country which has its own reasons for opposing Iran, remains an important component in the American strategy, but the U.S. must also manage the political costs of this relationship as it works with the Sunni Arab states. American opposition to Iran's nuclear program not only reflects concerns about Israeli security and the possibility that Iran might supply terrorist groups with nuclear materials. It also reflects the U.S. interest in protecting its ability to project conventional forces into the Gulf.
And what would happen if we left:
The end of America's ability to safeguard the Gulf and the trade routes around it would be enormously damaging -- and not just to us. Defense budgets would grow dramatically in every major power center, and Middle Eastern politics would be further destabilized, as every country sought political influence in Middle Eastern countries to ensure access to oil in the resulting free for all.
The potential for conflict and chaos is real. A world of insecure and suspicious great powers engaged in military competition over vital interests would not be a safe or happy place. Every ship that China builds to protect the increasing numbers of supertankers needed to bring oil from the Middle East to China in years ahead would also be a threat to Japan's oil security -- as well as to the oil security of India and Taiwan. European cooperation would likely be undermined as well, as countries sought to make their best deals with Russia, the Gulf states and other oil rich neighbors like Algeria.
As tempting as the idea of picking up our toys and coming home sounds, such a course would not make us more secure in the long run and it could be disastrous for the global economy. The reality is that a country (or some combination of them) is going to project power in the Gulf. While doing so is a significant burden for the US to bear, the alternative of leaving it to someone else would be far more costly.