A US anti-missile base in Poland will become operational on Friday, the Polish government has said.
The Redzikowo Air Base in the north of the country will be the second US base of its kind in Europe, the first being in southern Romania.
The American-built Aegis system deployed on both sites is intended to protect against ballistic missiles from Iran, according to Nato.
Russia strongly opposes the bases, calling them a security threat.
Prime Minister Mateusz Morawiecki told parliament that the US had informed his government that the base would begin operations on 15 December.
The bases in Romania and Poland are part of a missile defence system which also comprises a radar facility in Turkey, a command centre in Germany and US Navy ships.
The Redzikowo base was intended to begin operating in 2018 but its opening has been repeatedly delayed.
The Aegis system is designed to detect missile launches with satellite systems. Once identified, interceptor rockets are launched from sea or land at the enemy missile, destroying it in space.
The alliance says the system will protect members against short- and medium-range ballistic missiles launched from Iran. The system does not work against cruise missiles, however.
Russia has complained that Aegis represents a threat to its nuclear deterrent – a claim denied by Nato.
The Redzikowo base is located around 150km (93 miles) from Kaliningrad, a Russian exclave on the Baltic sea between Poland and Lithuania.
The Aegis system is also deployed on American, Japanese and South Korean ships in east Asia.
There it is seen as intending to protect against Chinese and North Korean missiles aimed at US allies in Asia.