US military expenditures are about 40% of global defense expenditures. In 2012 US military expenditures were about $682 billion. This is more than the amount spent by the next 13 highest country spenders combined.
The war is now over in Iraq for the US, and by the end of 2014 the main body at least of US troops will be withdrawn from Afghanistan. The Iraq and Afghan wars were very costly. Even though there will still be expenditures by the US in the two countries, it should be much reduced, producing a possible peace dividend.
If the defence budget were cut substantially, the money saved could be used for pressing domestic problems and also to reduce the deficit. Yet there is very strong resistance towards any very substantial cuts to defense spending. This is because, private military contractors, the Pentagon, and Congress all benefit from keeping spending well above what may be needed. Military contractors earn profits from the spending, the Pentagon gets the weapons they want and further their power within the system, and finally, politicians provide jobs for their constituents and bring funds into their districts helping to ensure their re-election. The result of all this is that in the battle to ward off cuts, the military-industrial-complex will be able to fend off severe cuts and the amount of the peace dividend will shrink drastically.