Trump, who once famously declared NATO to be "obsolete" only to later say is was "no longer obsolete," has disrupted the status quo within the organization over the last three years. His most persistent complaint is that the U.S. is footing more than its fair share of the bill, while calling on other members to increase their spending.
But he is not the only leader attending this week's summit to have upset the status quo.
French President Emanuel Macron recently sent shockwaves through the alliance by making a provocative warning of NATO's "brain death," while Turkey's President Recep Erdogan acted out of accord by invading Northern Syria and with his purchase of Russian military equipment.
Trump, for his part, has made strides toward his goal of reducing the U.S. financial commitment to NATO.
"The president has been spectacularly successful," a senior administration official said in a call with reporters. "In 2016, only four allies spent 2% of GDP on defense. Now, there are nine, and following through their implementation plans to get the 2%, we expect there to be 18 by 2024. This is tremendous progress, and I think it is due to the president's diplomatic work."
Just last week, it was announced that the U.S. is reducing its contribution toward NATO's operational budget from 22% to 16%. A NATO official portrayed the move as a demonstration of the allies' commitment to the alliance rather than a sign of decreased U.S. commitment.
"Allies regularly review cost shares for NATO's common funding. All allies have agreed a new cost sharing formula," a NATO official said. "This is an important demonstration of allies' commitment to the alliance and to fairer burden-sharing."
While many have interpreted Trump's emphasis on costs as a sign of a waning U.S. investment, a senior Trump administration official argued that his real motivation is a desire to make NATO stronger.
"The president, as you know, has been committed to making NATO stronger and ready to face today's threats and tomorrow's challenges. This is why he places such an emphasis on encouraging all allies to live up to their commitments and increase defense spending, in line with their Wales commitments," the official said.
But while the administration contends the president has only made NATO stronger, Macron made headlines last month with his warning of a "brain death" within NATO because of reduced U.S. reliability within the structure of the alliance.
Macron told The Economist magazine that the U.S., under Trump, appeared to be turning its back on Europe and called on Europeans to do more in their own defense.
Those comments drew a rebuke from German Chancellor Angela Merkel, who according to the New York Times privately scolded the French leader.
"I understand your desire for disruptive politics. But I'm tired of picking up the pieces. Over and over, I have to glue together the cups you have broken so that we can then sit down and have a cup of tea together," Merkel reportedly told Macron, according to the New York Times.
Heather Conley, the director of the Europe Program at the Center for Strategic and International Studies, said it seems as if Trump and Macron are "trying to out-disrupt the other."
"The political level NATO has certainly been in great turmoil since President Trump's election, and now, as we add President Macron into this, now this is starting to feel a little more unstable," Conley said.
Macron, for his part, has been pushing for NATO to undertake a review of its Strategic Concept, a complicated and lengthy process to update the official document governing NATO's purpose.
While Trump and Macron have their own distinct agendas for reform within NATO, it will be a closely watched moment when the two leaders sit down for a bilateral meeting in London.
"They have a great deal of respect for each other," said a senior administration official. "They have different priorities for the alliance. The president wants to make it stronger and the burden sharing more equitable. I think President Macron is still kind of working out what he wants out of the group. But I think they have a healthy level of respect for each other. That will come out in their bilateral conversation."
The president is also scheduled to sit down for a bilateral meeting Merkel, with whom he has at times had a strained relationship.
And while Macron may have made provocative comments, the most provocative actions of late have been taken by Turkey's president with his invasion of northern Syria. The move was not supported by NATO and has created a dangerous scenario under which Turkey opened itself up to attack.
"Turkey may have by its own decisions -- without consultation -- placed itself in such risk that should it be attacked and should it turn to NATO for protection, you could have -- you know -- the nightmare, the worst case scenario for NATO, where a member seeks, hypothetically, an Article 5 commitment, and then the alliance does not approve or does not agree to that," Conley said.
In addition to its actions in Syria, Turkey's increasingly close ties with Russia and its purchase of a Russian missile defense system remains a source of widespread concern within the alliance. A senior administration official said the U.S. has already been "blunt" with Turkey on the issue of the Russian S-400s, but so far the administration has stopped short of sanctioning Turkey over acquiring the Russian equipment, despite legislation that mandates the administration to do so.
Trump is not scheduled to sit down with Erdogan for a one-on-one meeting, a senior official said, since the Turkish leader was just recently at the White House and engaged directly with the president at that time.
Despite the various causes of turmoil at the leadership level within the NATO alliance, Conley said the "nuts and bolts" operations of NATO remain effective, and she pointed to Russia's incursion into Ukraine as a unifying and transformative moment in NATO's recent history that helped to strengthen the alliance by "returning NATO to its founding mission."
She also noted that this is hardly NATO's first moment of internal crisis, but said the key ingredient in previous resolutions has been strong U.S. leadership.
"NATO has undergone identity crises since its founding, usually about every decade or so, it really undergoes some, some strains, but it's always been resilient in finding its way through those challenges. And a lot of that has depended on U.S. leadership," she said.
The politics of NATO aside, this week's meeting comes at a critical political moment for the U.K. domestically just days ahead of a general election.
Trump is a close political ally of Prime Minister Boris Johnson, but Conley said the president's presence on Johnson's turf just ahead of the election is a "double-edged sword" for the prime minister.
It is still not clear whether Johnson and Trump will sit down for a bilateral meeting but the two spoke by phone over the weekend ahead of the president's visit.
A senior administration official said the president is "very conscious" of sensitivities around not wading into other countries' domestic politics, but the president has been known to break that norm and freely offer his opinions on UK politics on more than one occasion in the past.
Even if the president is not ultimately received for a bilateral meeting with the prime minister hosting the meeting, he and other world leaders will receive a royal welcome when the queen hosts them for evening reception at Buckingham Palace.