WARSAW, Poland - The United States has entered a decisive phase in a plan to set up missile defense sites in Eastern Europe — a system Washington says is aimed at protecting itself and its allies against potential attacks from the Middle East. But the prospect of sophisticated U.S. radar and interceptor systems in formerly communist Eastern Europe has led Russian military leaders to warn of a new arms race. The system "would create a clear threat for Russia," Col. Gen. Vladimir Popovkin, the chief of Russia's Space Forces, warned Monday. The United States told Polish leaders it wants to...