The United States and Poland agreed on August 14 to build the long-discussed U.S. missile interceptor base in Poland. Senior Russian general Anatoliy Nogovityn responded by threatening to attack Poland, possibly with nuclear weapons. (This is the same fellow who assured the Georgians on August 4 that Russia was not planning an offensive.)
Longstanding Russian opposition to this missile defense system is political, not technological. The system aims to intercept missiles from sources such as Iran, and is not powerful enough to degrade Russian offensive missile capabilities. The Russians are unhappy, though, to have a U.S. military presence on the territory of a former Warsaw Pact member.
The Russians' success in Georgia may be encouraging them to bully the next country in line. The dynamics this time should be different, though. Poland is a member of both NATO and the EU. It is bigger and stronger than Georgia, its common border with Russia is much shorter, and there is no remaining element of surprise.