Poland joined NATO in 1999 along with Hungary and the Czech Republic, and since then other former communist states, including Baltic republics bordering Russia, have joined despite Moscow's strong opposition.
Poland has repeatedly requested a permanent U.S. military presence on its soil. The United States currently rotates troops through Poland temporarily but permanently stationing forces there would be expensive because of costs that can include housing for families, schools and hospitals.
The plan would deploy 15,000 US troops and 250 tanks and armored vehicles in Poland.
Any decision to base such a force in Poland would be seen in Moscow as a serious breach of the 1997 Nato Russia Founding Act, through which Nato agreed not to deploy permanent forces in eastern Europe as part of its expansion plans.