There is an interesting historic president. When the UK realised it could no longer compete with the US and Germany simultaneously in the early 1900s and abandoned its two navies policy of being bigger than the next two navies combined.On Thursday, Panetta is expected to announce the results of a Pentagon strategic review, recommending that the US abandon its traditional goal of being able to fight, and win, two wars at the same time. The Pentagon has been asked for cuts of $400bn.
America does however have a very soft bed to fall back into. The worlds largest economy the EU and other industrial giants like S Korea and Japan are firmly comitted allies due to need.
America also now lacks the appetite for a bloody conflict. A huge amount of the cost of Iraq was in avoiding US casualties and if the money is not their then the political will will not be there.
Even with a scale back the total NATO defence estabilshment will be huge.
In a tinge of irony the only empire I can think of that scaled back into the protective cover of so strong an ally was the UK in the post WWII era when it was such a close ally of the US. 30 years before there was a widespread belief they were about to go to war over a new naval race! Hence the Washington treaty of 1922 (or there abouts). Britain simply inceasingly handed over comitments to the US as it could no longer afford them. Most significantly in the Gulf of Arabia during Harold Wilsons time as prime minister. From the 19th century until the 60s the non Turkish states had looked to the UK to protect them and afterwards the US enthusiastically replaced them.
Things like that guarentee are not going anywhere quickly. Not while the oil flows.
Tempus fugit.
A more multipolar world but not the end of an Empire just yet.