Poland has hailed the installation of a US Patriot surface-to-air missile battery, amid criticism from Russia.
Poland said the missile launchers, positioned about 60km (40 miles) from the Russian enclave of Kaliningrad, would enhance its security.
The battery will be rotated in and out of Poland for the next two years, along with dozens of US troops who will train Polish counterparts to operate it.
It is the first US deployment of its kind in Poland, officials say.
The short-range Patriot missile battery was delivered to a military base at Morag on Sunday.
A spokesman for the US army in Europe was quoted as saying that actual missiles would not be deployed at Morag, though the US ambassador was quoted as saying they might be.
A Russian foreign ministry statement on Wednesday criticised the move.
"Military activities like these do not contribute to strengthening our common security, developing relations of trust and predictability in the region," said the statement, which was carried on Russia's Itar-Tass news agency.
"We have more than once stated that we do not understand the logic and focus of US-Polish co-operation in this sphere," it said.
The statement also said Russian questions to the Poles and Americans had remained unanswered, as had their "arguments in favour of moving the area of temporary deployment farther from the Russian borders".
But US and Polish officials said the missile battery was purely defensive.
"You don't need to be a specialist to know that this kind of defence weapon cannot be turned into an offensive weapon," said Polish Defence Minister Bogdan Klich.
He said the project would encourage "the enhancement of Poland's security and the construction of co-operation and trust between Poland and the United States".
The Patriot missile deployment was originally planned under former US President George W Bush.
His administration agreed to a Polish request for the battery along with a broader missile defence plan that angered Russia.
US President Barack Obama scrapped the plan - a move that was welcomed by Russian officials - and announced his own, reconfigured missile defence policy.
Under the Bush administration plan Poland had been due to host 10 long-range interceptor missiles. It is now expected to host a smaller-scale defence base with short-range missiles.