The possibility of a permanent U.S. military base in Poland moved a few steps forward this week.

U.S. Defense Secretary Jim Mattis met with his Polish counterpart Mariusz Blaszczak Tuesday to discuss Poland’s offer to spend $2 billion on a permanent base for U.S. troops and their families in the country. The base was jokingly called “Fort Trump” by Poland’s president after meetings with U.S. President Donald Trump.

“We are working out the details on what the offer is, what can best contribute to alliance unity, to alliance security and to the continued strengthening of the bilateral relationship,” Mattis told the press prior to his talks with Blaszczak.

Mattis also praised the Polish government for meeting NATO’s defense spending commitments prior to heading into discussions with other Pentagon officials to discuss the potential base.

“It’s obviously something we’ll discuss with all of the allies as well,” Mattis added. “We have a very strong bilateral relationship with the Polish military, and the minister and the ministry of defense, and we’ll further discuss it when we’re in Brussels with them.”

An F-22 Raptor from the 95th Fighter Squadron, 325th Fighter Wing, Tyndall Air Force Base, Fla., participates in a multi-aircraft flyover in Warsaw, Poland during the 100th Anniversary of Polish Independence and Armed Forces Day. (Senior Airman Joshua Magbanua/Air Force)

The details of Poland’s offer are still being worked out, Mattis said. He noted that officials are looking at what can best contribute to the NATO alliance’s unity and security, not just the U.S.-Poland bilateral relationship.

“We strongly appreciate the U.S. forces' presence in Poland, strengthening deterrence and defense of Poland and the whole NATO alliance," Blaszczak said after meeting with Mattis. "I hope it will transform in the coming years into an even more robust and long-term commitment.”

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Trump showed interest in the idea of a permanent U.S. base in September, particularly because of Poland’s financial contribution to it.

“Poland is willing to make a very major contribution to the United States to come in and have a presence in Poland,” Trump told reporters at the White House after he met with Polish President Andrzej Duda. "If they’re willing to do that, it’s something we will certainly talk about.”

Air Force Reapers are now flying ISR missions from Poland The U.S. Air Force’s MQ-9 Reaper drone quietly started flights from Miroslawiec Air Base, Poland, in May, but the service isn’t exactly saying why.

Poland has sought a permanent U.S. presence in their country since President George W. Bush was in office. That administration entertained the possibly of stationing ground-based interceptors in Poland as part of its missile defense system.

Those talks eventually fell through.

However, since the 2014 Russian intervention in Ukraine’s Crimea peninsula and Donbass region, Poland has played a larger role in the U.S. military’s deterrence initiative in Eastern Europe.

Poland has repeatedly hosted the 173rd Airborne Brigade Combat Team to demonstrate rapid deployment capability in the case of an invasion on NATO’s eastern flank, and the alliance’s Joint Force Training Center is already headquartered in the country.

In May, the Air Force also confirmed that the service’s MQ-9 Reaper drones now conduct intelligence, surveillance and reconnaissance sorties from Miroslawiec Air Base, Poland.

A copy of Poland’s proposal for the permanent base notes that the country is prepared to help with building schools and facilities for military spouses and children.

The proposal lists the country’s regions of Bydgoszcz and Toruń as possible locations for the hypothetical U.S. base. Those regions were selected based on their proximity to ranges, infrastructure, accommodations, and morale and welfare options for U.S. troops.

“Our troops that go there [to Poland] re-enlist in the U.S. military at a high rate, so they must enjoy it,” Mattis said Tuesday.