Since we don't get oil from Iraq, it might be asked why our military is there so big, and then the wide-angle lens has to be used. Europe and Asia get oil from the region, and we get some small amount from Saudi, but since oil is a world commodity, what China buys from the ME affects what we buy from Nigeria. So we do China's work and Europe's work in protecting the oil. Why should we do their work? The oil is that important to us that if waited for China and France/Germany to pick up their share of the workload we would find our oil prices escalating beyond the comfort zone. The free ride is in China and Europe, but since we are the big piece of the world economy we have to do this to remain the big piece.
Why should we do their work?
This is an interesting question that I've wondered about myself. Part of the reason is that oil is pegged to the dollar. In order for countries to buy oil they have to pay for it in dollars--and that includes china.
What would bankrupt the USA in a day would be if major oil producing countries decided to peg their oil in another currency. This is something that major minor oil producers like like Iran and Venezuela are already trying to do.