STUTTGART, Germany -- President Donald Trump cast new doubt on his willingness to come to the defense of NATO allies Tuesday when he said a small member state could provoke a conflict that could start "World War III."
Trump, during an interview on a Fox News talk show Tuesday, was asked by host Tucker Carlson why his son should come to the defense of tiny Montenegro, NATO's newest member.
"I understand what you are saying," Trump said. "I've asked the same question."
Last week, Trump gave his backing to NATO after a contentious meeting of heads of state in Brussels, where the U.S. president reportedly threatened to go it alone if members didn't spend 2 percent of their respective gross domestic products on defense.
By the end, Trump said he was satisfied with the commitments made by members and that NATO was now "much stronger" and a "fine-tuned machine."
Still, doubts have lingered among NATO watchers about Trump's underlying commitment to the alliance's Article 5 collective defense provision that an attack on one member equals an attack on all.
"Montenegro is a tiny country with very strong people," Trump said. "They are very aggressive people. They may get aggressive and congratulations you are in World War III."
Despite its diminutive size -- slightly smaller than Connecticut and with just 630,000 people -- Montenegro has a long martial tradition, having fought off the Ottoman Empire during its 400-year rule of the Balkans and having participated in both world wars on the winning side.
Trump, during a NATO meeting in 2017, drew attention when he elbowed the prime minister, Dusko Markovic, out of the way during a ceremonial walk at the alliance's headquarters. Markovic appeared taken aback at first but then patted Trump on the back.
Opponents of extending NATO membership to small nations in Eastern Europe contend that adding more security consumers, rather than security providers, adds to tensions with Russia.
Mark Hertling, the retired former commander of U.S. Army Europe and frequent critic of the current administration, said Trump's latest comments are eroding trust inside NATO.
"Stunning," Hertling said in a Twitter posting. "The 'provisions' are what makes it a mutual security and defense alliance. Alliance trust is gained in drops and lost in buckets."
Trump has long expressed ambivalence over defending allies. Early in his tenure, he suggested U.S. security guarantees depended on whether allies where spending enough money on their militaries.
For NATO, Article 5 serves as the linchpin to the security pact. U.S. commanders and Defense Secretary Jim Mattis have repeatedly emphasized that the American commitment to it is "ironclad."
But mixed messages from Trump create doubt, said Jorge Benitez, a NATO expert with the Atlantic Council.
At NATO's summit last week, Trump's confrontational approach with allies "weakened the alliance and damaged US national security," Benitez said.
"The only winner is (Russia President Vladimir) Putin. Putin may now question if NATO's Article 5 is still credible and he may be tempted to probe NATO and test the commitment of its members," Benitez told Stars and Stripes.
Allies in the Baltics are among the members most concerned about potential Russian aggression. On Tuesday, Estonian officials accused Russia of intruding on its airspace when Putin's jet crossed into Estonia en route to his meeting in Helsinki with Trump.
Estonian authorities called the brief incursion an illegal border crossing.
In the case of Montenegro, a leading candidate for membership in the European Union and an up-and-coming tourist destination among Europeans, Moscow is already suspected of meddling.
In 2016, Russian military intelligence was accused of having a hand in a failed coup attempt inside Montenegro by Serb ultra-nationalists opposed to NATO membership.
Moscow has long opposed NATO expansion eastward, which it sees as a security threat.
Inside NATO, there also has been some disagreement among members over the alliance's open-door policy to aspiring partners who meet NATO standards, with some members saying it is needlessly provocative toward Russia.