No country (except crazy ones like NK) spends billions of dollars on arms just because they think its the thing to do.. They do it mainly because they feel insecure due to the threat of foreign powers.
China didn't start its nuclear program until the US threatened to nuke them if they got involved in Vietnam , Iraq and Iran wanted weapons of mass destruction because of the threat of US invasion as well as the regular wars in the middle east.
And i'm not talking about having NO armed forces.. I'm talking about halving the currant numbers to the point where the US military is mainly defensive , and can probably fight only one large scale war overseas .
Tanks and fighter jets don't stop Al Queda , in the case of a 2nd 9/11 you only need one F16 to shoot down those civilian planes.
They still have thousands of nuclear missles , the USAF still operates 6000 aircraft , most of which never see service in combat situations , the US Navy has over 12 aircraft carriers with more under construction , the chinese have zero!
Actually according to wikipedia , small scale cutbacks are taking place :
The USAF is currently planning a massive Reduction-in-Force (RIF). Because of budget constraints, the USAF will reduce the service's current size by 40,000 full time equivalent positions by 2011, with approximately half to be eliminated in FY 2007. Approximately 35,000 active duty positions, or one year's cycle of enlistments and retirements, will be eliminated over 5 years.[3] The current size of the active-duty force is roughly 70% of that of the USAF at the end of the first Gulf War in 1991.[4]
http://en.wikipedia.org/wiki/USAF#Aircraft The reality is that eventually the US may not be able to afford to maintain large forces if their economy continues to falter , and all that money could be spent and put to better use.
Nuclear Fusion promises to bring near unlimited power to the industrialized world , the only drawback is the huge cost of R&D , 10 billion US for a single test reactor , but if money was spent to make it a reality dependence on fossil fuels will be a thing of the past.
The US population is morbidly unhealthy and obese , their productivity is less than most other nations and average life span is supposedly decreasing.. Cuba has lower infant mortality than the states .
Cures for AIDS , Cancer , colonies on the moon... all of it is only a matter of funding , and these things are more important than weapons that spend their lifetimes locked up in a warehouse.
Remember i'm not advocating we throw away ALL weapons , just a decrease to spend the money on something sensible .
And in the case of another gulf war , or world war III , if the US didn't act like jackasses , they would have help from other nations , the European Union , Canada , and occasionally Russia can more than take up the slack to defend the world against rogue states.
The main reason why so many countries continue to increase their military spending is that they feel threatened by US imperialism , of course they won't throw away their guns if that threat was gone , but it would result in a saner world i believe.