One of the key platforms of Dick Cheney's George Bush's defence policy has been the construction of a National Missile Defence, a system of ground-based interceptors designed to intercept nuclear missiles heading towards the United States. While ostensibly aimed at "rogue states" such as North Korea and Iran, neither actually have missiles capable of reaching the continental US (though North Korea is close), and other nuclear powers such as Russia believe that it is really aimed at them - an impression enhanced by the US's desire to station radars and interceptors in the Czech Republic and Poland.
Russia is concerned that a US anti-missile system will effectively counter their nuclear deterrant (a serious concern, given the weak state of the Russian nuclear forces), rendering them (more) vulnerable to a pre-emptive nuclear strike. Which isn't a situation they want to be in. So, they've decided to up the diplomatic ante by suspending their participation in the Treaty on Conventional Armed Forces in Europe, suspending inspections and the exchange of data. This doesn't sound like much, but underlying it is a threat to deploy more forces in the Kaliningrad enclave, directly threatening Poland and allowing them to strike at the missile defence bases. Needless to say, this isn't particularly good for international security, and its particularly bad for Europe, which once again gets to bear the brunt and be the playground for America's foreign policy squabbles.
As with the "war on terror", Bush's efforts at improving the security of Americans have in fact exported insecurity and made the world a much more dangerous place for everyone else. Thanks, George!