President Trump renewed his criticism of European allies on Tuesday and said he was most optimistic about meeting with President Vladimir V. Putin of Russia, as he departed for a summit meeting in Brussels that threatens to highlight acrimony within NATO.
Mr. Trump’s comments touched off a round of trans-Atlantic sniping with Donald Tusk, the president of the European Council, that was sure to start the NATO meeting on a tense note.
“The U.S. is spending many times more than any other country in order to protect them,” Mr. Trump said on Twitter of the other members of the Atlantic alliance, hours before Air Force One left for Belgium. “Not fair to the U.S. taxpayer.”
He complained anew about trade deficits with the European Union, and seemed to threaten to cut American military spending in a bid to compel other NATO members to increase theirs.
“NATO countries must pay MORE, the United States must pay LESS,” Mr. Trump said in a separate tweet. “Very Unfair!”
The complaints reflected the degree to which Mr. Trump is coming into the summit meeting focused on his anger with NATO and his conviction that the alliance exploits American largess to the detriment of the United States. That is a stark departure from previous American presidents of both parties, who have tended to regard the alliance as an invaluable force for collective defense that reflects shared values among its members.
Mr. Trump’s remarks exacerbated concerns that he may torpedo the meeting that begins on Wednesday, and with it, the alliance’s efforts to show unity and solidarity in the face of global threats, including from Russia.
“Frankly, Putin may be the easiest of them all — who would think?” Mr. Trump said of his discussions over the next week, which include the NATO gathering, a working visit on Friday with Prime Minister Theresa May of Britain, whose government is in turmoil; and his first summit meeting with the Russian president on Monday in Helsinki, Finland.
Mr. Trump’s tweets prompted a tart retort from Mr. Tusk, who answered back on Twitter saying that Europe was a friend worth protecting, unlike other countries with which Mr. Trump has cultivated relationships.
“Dear @realDonaldTrump,” Mr. Tusk wrote. “US doesn’t have and won’t have a better ally than EU. We spend on defense much more than Russia and as much as China. I hope you have no doubt this is an investment in our security, which cannot be said with confidence about Russian & Chinese spending.”
In comments following the signing of a joint European Union-NATO declaration, Mr. Tusk said he wanted to address Mr. Trump’s near-daily criticism of Europe and complaints that countries are “living off the U.S.”
“Appreciate your allies,” Mr. Tusk said. “After all, you don’t have that many.”
Mr. Tusk, the president of the body that represents leaders of European Union member states, many of which are also in NATO, has a history of publicly challenging Mr. Trump, at a time when many foreign leaders are wary of the American president but reluctant to criticize him openly.
Last month, he said that Mr. Trump’s politics had put trans-Atlantic relations “under tremendous pressure” and warned Europeans to prepare for darker times. In May, he tweeted that Mr. Trump’s decisions could prompt the question, “With friends like that, who needs enemies.”
On Tuesday, as he left the White House for the short helicopter ride to Andrews Air Force Base to begin his trip to Europe, Mr. Trump seemed unmoved by Mr. Tusk’s latest pushback.
“Well, we do have a lot of allies, but we cannot be taken advantage of,” the president said, when asked about Mr. Tusk’s comments. “We’re being taken advantage of by the European Union.
“We lost $151 billion last year on trade, and on top of that we spend at least 70 percent for NATO, and frankly it helps them a lot more than it helps us. So we’ll see what happens.”