Poland’s US anti-missile facility will start operations on Friday, the country’s government announced.
The Redzikowo Air Base in the northern part of the country will be the second US base like this in Europe. The first one is in southern Romania.
Nato says the Aegis system, built by Americans and used at both sites, is meant to defend against ballistic missiles from Iran.
Russia does not like the bases and thinks they are dangerous for security.
Prime Minister Mateusz Morawiecki said in parliament that the US government told his government that the military base would start working on 15th December.
The bases in Romania and Poland are part of a system to protect against missiles, which also includes a radar facility in Turkey, a command center in Germany, and US Navy ships.
The Redzikowo base was supposed to start working in 2018, but it has been delayed many times.
The Aegis system can find when missiles are launched using satellites. Once we find the enemy missile, we shoot rockets at it from the sea or land to destroy it in space.
The alliance says the system will keep its members safe from missiles launched from Iran. The system cannot stop cruise missiles.
Russia says Aegis is a danger to its nuclear weapons, but Nato disagrees.
The Redzikowo base is about 150km (93 miles) away from Kaliningrad, which is a part of Russia and is surrounded by Poland and Lithuania.
The Aegis system is used on ships in east Asia from America, Japan, and South Korea.
It is meant to protect US allies in Asia from Chinese and North Korean missiles.