Allies and Former U.S. Officials Fear Trump Could Seek NATO Exit in a Second Term

“It is a real risk,” said Thomas Wright, director of the Center on the United States and Europe at the Brookings Institution, a think tank in Washington. “We know from Kelly and Bolton that he wanted to go much farther in the first term. If he feels that he has been totally vindicated in the election, and he feels that people have endorsed his policies, I think he could effectively withdraw from NATO.”

Congress would most likely move to block any effort by Mr. Trump to exit the alliance altogether, but experts said he could deal it a near-lethal blow in other ways. One would be to undermine a provision in the original treaty, Article 5, which calls for collective self-defense and which every past president has interpreted as a promise to defend any member from military attacks, but which Mr. Trump has questioned in the past.

“He could just reinterpret it as, ‘I could just send a strongly-worded letter,’” Mr. Wright said.

Jorge Benitez, a senior fellow at the Atlantic Council, a think tank in Washington, noted that the Trump administration announced plans in July to withdraw 12,000 American troops from Germany, the strategic heart of the alliance, and sought to cut funding for the Pentagon’s European Deterrence Initiative, a program whose funding the administration initially called for increasing and pointed to as evidence of the president’s support for the alliance.

European officials, Mr. Benitez said, “see the escalation of negative steps, and they are definitely concerned that that negative pattern could continue if Trump is re-elected.”

On Capitol Hill, Democrats focused on security issues say a re-elected Mr. Trump could permanently reshape the relationship between America and Europe, which has been defined for generations by Washington’s bipartisan role as a leader and protector of the continent.

“Withdrawing from NATO would be nothing short of catastrophic and further highlights the historic importance of this election,” said Senator Jeanne Shaheen, Democrat of New Hampshire.

“Bipartisan support for NATO in Congress is unwavering and overwhelming, and there are significant procedural hurdles if any President were to choose this path,” added Ms. Shaheen, a senior member of the Senate Foreign Relations Committee. “President Trump has undermined trans-Atlantic relations from day one, and the only one reaping the benefits is Vladimir Putin. Speculation of a future withdrawal is in itself a victory for the Kremlin and beyond Putin’s wildest dreams.”