President Donald Trump celebrated “another win” on Monday by announcing the U.S. State Department had helped broker direct airline flights between Serbia and Kosovo.

“Everyone said it couldn’t be done!” the president declared on Twitter, applauding White House National Security Adviser Robert O’Brien and U.S. Ambassador to Germany Richard Grenell for making it happen:

Germany’s Deutsche Welle agreed the deal was a “diplomatic milestone,” assuming everything is implemented as agreed:

The deal, brokered by the United States in an effort to normalize relations between the two countries, would see the first flights between the two since 1998, when war broke out in Kosovo between Serbian security forces and ethnic Albanian insurgents, aided by NATO and Albania.

The air link between Belgrade and Pristina is set to be provided by Eurowings, German Lufthansa Group’s low-cost subsidiary. However, there were no details on when the flights would begin. The distance between the cities is around 520 kilometers (323 miles) and takes around 5.5 hours by car.

The deal, signed on Monday at the US Embassy in Berlin, “marks another step in our efforts to normalize relations between Kosovo and Serbia,” said Kosovo President Hashim Thaci in a statement.

Ambassador Grenell, who served as a special envoy between Serbia and Kosovo while the air travel deal was negotiated, said President Trump “once again led us to an historic victory” by moving Serbia closer to formally recognizing Kosovo. 

“What we heard from the business community in Kosovo and from the business community in Serbia is we want a more normalized commerce structure and we need a flight. It’s the reason why we went after trying to figure out how do we make a direct flight for the first time in a generation,” Grenell told the New York Times on Tuesday. 

Grenell also thanked Serbian President Aleksandar Vucic and Serbian Prime Minister Ana Brnabic for their “commitment and friendship,” predicting the airline deal would “create jobs and open new markets” for both Serbia and Kosovo. Vucic responded that Serbia is “ready to pursue more such initiatives, bringing people in the Balkans closer together.”

“We are grateful to President Trump for the continued attention and care that he demonstrates towards Kosovo. We know that we are on the right track in any process when we have U.S. support,” Kosovo parliamentary speaker Kadri Veseli said when the deal was announced.

Kosovo formally declared independence in 2008. Serbia, Russia, China, and most of their allies do not recognize Kosovo as independent, while the United States and most of its allies do. Russia has been blocking Kosovo from obtaining membership in the United Nations as an independent state.

There are a few hitches that could keep the deal from reaching full implementation, beginning with the possibility that Kosovo’s government could collapse. The current president, Hashim Thaci, has not been able to form a stable government since Prime Minister Ramush Haradinaj resigned in October over allegations of war crimes committed during the Balkan wars in the late 1990s.

Thaci technically has less than two weeks left to put a government together in order to stave off new elections, which could conceivably install an administration less enthusiastic about the air travel deal with Serbia. 

Part of the deal involves Thaci lifting 100 percent tariffs that were imposed against Serbian goods by Haradinaj, a move that antagonized both Serbia and the large number of ethnic Serbians living in northern Kosovo. The tariffs were a response to Serbia blocking Interpol membership for Kosovo. Kosovo withdrew its application in October but is expected to try again under the new government in 2020. 

NATO Secretary-General Jens Stoltenberg said the flight deal is an “important step” because it will “make the circulation of people and goods easier and faster within the Western Balkans region.” 

The lingering question will be whether such circulation breaks up diplomatic obstacles to Serbia and Kosovo normalizing relations, in addition to making open conflict less likely. The Serbian government has made encouraging noises about negotiating on numerous outstanding issues once Kosovo’s tariffs are lifted.