Poland has announced a deal to buy US Patriot surface-to-air missiles, amid rising tension with Russia.
President Bronislaw Komorowski said Poland would start talks with the US to finalise the multibillion contract.
The move follows reports that Russia has deployed missiles in its exclave of Kaliningrad, which borders Poland.
Russia’s annexation of Crimea last year and its support for rebels in eastern Ukraine have caused great concern among Nato states, notably in Eastern Europe.
Russia denies accusations by Nato that it is arming the rebels and sending troops into Ukraine.
The Patriot is an advanced missile system intended to defend against aircraft, cruise and ballistic missiles.
“For the armed forces’ technical modernisation, and the Polish armed forces’ resilience, to be effective, the so-called anti-missile shield… had to become the priority of priorities”, President Komorowski said on Tuesday.
He added that regional security had worsened because of the conflict in Ukraine.
Warsaw wants to buy the Patriots because it is concerned about Moscow placing missiles in Kaliningrad that borders northern Poland, the BBC’s Adam Easton in Warsaw reports.
Poland currently has no defence against such weapons.
The missile deal – which could be worth about $7bn (£4.7bn) – is the largest in Polish history, our correspondent says.
It is part of Poland’s plans to spend $35bn to modernise its military over the next eight years.
Poland also plans to buy helicopters, submarines and armoured personnel carriers.