I'm currently re-reading
The Heritage Trilogy by Ian Douglas -
http://en.wikipedia.org/wiki/The_Heritage_Trilogy . The timeline of the series starts in 2040 and the premise behind it is that the USA has withdrawn from the UN in protest at the UN trying to force a plebiscite within the southern states of the USA and the northern states of Mexico over the formation of a new Hispanic nation to be called the Aztlan Republic. The USA is still the world's dominant superpower, especially in space and on the high seas, and it is closely allied with Russia which has also withdrawn from the UN in protest at the UN's attempt to force it to negotiate with China over ownership of parts of Siberia.
One reason in particular why I like the series is that it focuses on the USMC which, in the decades before the first book starts, has been fighting for its survival as bean-counters in the US government try to win support for the Corps' disbanding. When alien artifacts are discovered on Mars and the UN tries to use military force to take over the archaeological sites on Mars the USMC gains a new lease on life by successfully countering the UN's forces and taking back control of America's Martian holdings. In the series the UN's space power is provided by the ESA, China and Japan's space agency. China is very much a world power in the series but is still no match for the US.
The premise of the series does make me chuckle a bit because it plays on the idea that the UN is out to undermine the US and make itsself into a world government (I understand that many Americans IRL take that idea pretty seriously). Anyway, the series is a good read and it provides food for thought about future roles the USMC might have in offensive operations in space or on other planets. I would urge past and present Marines and their friends and admirers to check this series out. Here is the link to the Wikipedia article on the first book in the series -
http://en.wikipedia.org/wiki/Semper_Mars .