The Brookings Institution. July 2012.

There have long been debates about the sustainability of the transatlantic alliance and accusations amongst allies of unequal contributions to burden-sharing. But since countries on both sides of the Atlantic have begun introducing new – and often major – military spending cuts in response to the economic crisis, concerns about the future of transatlantic defense cooperation have become more pronounced. [Note: contains copyrighted material].

http://www.brookings.edu/~/media/research/files/papers/2012/7/military%20spending%20nato%20odonnell/military%20spending%20nato%20odonnell%20pdf [PDF format, 30 pages].