WASHINGTON (CNN) - President Donald Trump capped off his dramatic appearance at the annual NATO summit in Brussels, Belgium, on Thursday by teasing another controversial possibility, saying he would not rule out ceasing NATO military exercises in the Baltic States if Russian leader Vladimir Putin requested it during their upcoming meeting in Helsinki, Finland.
Asked if he would cancel them, Trump said, "Perhaps we'll talk about that," referring to Putin, a response that has only fueled the the growing uncertainty over the state of the alliance.
Despite Trump's barrage of acidic rhetoric during the summit, diplomats said the President never explicitly threatened to leave NATO and it remains unclear whether he has the power to do so without Senate approval.
However, Trump's comments about possibly canceling NATO military exercises at Putin's request have amplified fears among European leaders that the President could undermine the alliance by making major unilateral concessions to the Russian President -- who he again referred to as a "competitor" rather than an adversary on Thursday.
One defense official told CNN Thursday that the Pentagon is not anticipating Trump suspending exercises in Eastern Europe.
In fact, joint exercises involving 17 nations -- including hundreds of US troops and several warships -- are currently underway in the Black Sea.
But the President's comments have certainly left the door open for Putin to broach the topic of military exercises in Helsinki and as commander in chief, Trump can make the decision to suspend US involvement on his own.
Trump demonstrated he is willing to exercise that power last month by abruptly suspending joint exercises with South Korea following his summit with North Korean leader Kim Jong Un in Singapore.
But canceling NATO exercises is widely viewed as a more extreme measure and a step that would fracture the notion of collective defense that is central to the alliance's identity and, Europeans worry, would serve as a major concession to Moscow.
Should he choose to pull US involvement, European nations could still conduct the military games, though they would likely be forced to do so under a different flag as the US can veto branding them as NATO exercises.
Barry Pavel, a senior vice president at the Atlantic Council, said Putin is highly likely to see whether he can get Trump to postpone or cancel NATO exercises set for Eastern Europe when they meet Monday in Finland.
"I guarantee you Putin will ask if we can stop NATO exercises, which he likely will characterize as provocative and expensive," Pavel said, noting that North Korea used the same gambit with success.
The outcome of the Trump-Putin meeting will affect perceptions of the 29-member NATO summit, Pavel said.
"Today was chapter one," Pavel said of the alliance meeting in Brussels. "Chapter two will play out next week in Helsinki. It's obvious that many allies are quite concerned about what will take place next week."
Republican Sen. John McCain, of Arizona, issued a stark warning to Trump ahead of his trip to Helsinki.
"The President's task is to reverse his disturbing tendency to show America's adversaries the deference and esteem that should be reserved for our closest allies. He must show that he can be strong and tough with Vladimir Putin -- not for its own sake, but to demonstrate his willingness to defend America, its allies, our shared interests, and our common values against those who threaten them," McCain said in a statement.
"Putin is not America's friend, nor merely a competitor. Putin is America's enemy -- not because we wish it so, but because he has chosen to be," he added.
Trump's own secretary of defense, James Mattis has also made his views on Russia and Putin very clear in recent months -- at times seemingly contradicting the President.
"He aims to diminish the appeal of the Western democratic model and attempts to undermine America's moral authority. His actions are designed not to challenge our arms at this point but to undercut and compromise our belief in our ideals," Mattis said last month at a graduation ceremony for the US Naval War College in Newport, Rhode Island.
While the summit was a success on an operational level, with significant agreements on readiness, new commands and cybersecurity, among other things, Pavel said he was concerned that Trump's behavior dealt the alliance a blow. "I worry that it is in some way tearing at the fabric ... of the transatlantic bonds," he said.
The alliance "is built on trust and is built on values," Pavel said. The amount and level of emotion heard from foreign leaders "was not a good sign."
"The US leads in the world by the power of attraction, not by the power of coercion," Pavel said. "Germany is not North Korea, so the same tactics the Trump administration used on North Korea should not be applied to the closest allies of the United States."
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