Former NATO Supreme Allied Commander James Stavridis said the return of President-elect Donald Trump to the global stage could produce "internal tension" within the alliance, in part over Trump's public positions about Ukraine.
Stavridis, NATO chief from 2009-2013, made the comments in an interview with MSNBC on Friday.
"There'll be a lot of internal tension within the alliance, and it'll stem from two things. One is increased pressure from the Trump administration for our European allies to increase their defense spending," Stavridis said.
"Secondly, ... there'll be tension about Ukraine and the way forward," Stavridis said.
"I think unfortunately for our friends in Kiev, they are in for a more difficult time getting what they need to continue to fight against [Russian President] Vladimir Putin."
Ukrainian President Volodymyr Zelenskyy in a speech Thursday said he had spoken with Trump after his election victory, calling it a "productive conversation, a good conversation."
"Of course, we cannot yet know what his actions will be. But we do hope that America will become stronger. This is the kind of America that Europe needs. And a strong Europe is what America needs, to my mind. This is the connection between allies that must be valued and cannot be lost," Zelenskyy said in the address he gave at a European summit in Budapest.
Trump and Zelenskyy met in September in New York, where the former president said he would not abandon Ukraine but wanted to give diplomacy a chance.
"I think we have a common view that the war in Ukraine has to be stopped and Putin can't win," Zelenskyy said at the time.
At a September rally in Pennsylvania, Trump said Zelenskyy hoped that Democrat Vice President Kamala Harris would win Tuesday's election.
"I think Zelenskyy is the greatest salesman in history. Every time he comes into the country, he walks away with $60 billion," Trump said then. "He wants them to win this election so badly, but I would do differently. I will work out peace."