In the US, there was a famous oligarch named John D. Rockefeller, who as a fatherless child grew to dominate the US oil industry with the Standard Oil monopoly, slowly purchasing all competitors or else pushing them from the market through aggressive business tactics such as buying all the barrels in a region so nobody else could use them.
In the opposition of monopolies, the US government established anti-trust laws to ensure fair competition in industry thus giving price control to the free market.
However starting around 1933, Standard Oil branched into world-wide oil exploration establishing ties with Saudi Arabia. During this time period, the oil industry in the middle east was non-existent, and its entire industrial infrastructure needed to be created, owned, and operated, mostly by foreign companies. Some of those companies combined to establish the Arabian American Oil Company, which was eventually sold with total control to Saudi Arabia since the industry was built entirely on their resources and so was rightfully theirs anyway. To the US this was actually a significant benefit because it ensured a stable and peaceful supply of oil.
However, with what concern for oil influenced the ending of Saddam Hussein's dominance over Iraq? If the United States or Russia wanted to control the mid-east oil, then why wait until now to do it?
At some point in the future the world oil supply will be empty. It is consumed at a rate faster than it is created near the earth's core. Both the US and Russia wisely regulates his own resources - each restricting consumption of domestic oil while pumping into strategic reserves to ensure that the "black gold" does not leak into vacuums created by depletion of other regions.
Meanwhile, both countries are looking into sources of alternative power such as hydrogen and deuterium. Therefore not only the existence of, but the need for mid-eastern oil in the future will simply fade away. Therefore, the creation of a military base in Iraq for the purpose of securing its oil stockpile would only yield a short-term gain at best. But does such a base give any more strategic advantage to a military where the soldier already has a long list of "resorts" on which to "serve his vacation?"
Something Bush has said might give some insight to the American mentality - that to plant a democracy in the heart of the Middle East could bring about a positive ideological change to the region. Those who accuse the US of conquest or monopoly simply do not understand this mentality.
The Americans came to realise that they would have to either leave the region or weaken their presence there. It is Russia that is filling the vacuum now