The unpredictable USA president - and his angry demand that allies such as Canada increase their defence spending - was very much the focal point during this week's gathering in Brussels, with memories of last month's disastrous G7 meeting in Quebec still fresh in the minds of many.
The agreement secured by the deal-maker president is apparently the very one the 29 member nations had formally agreed to the day before.
In 2011, when the USA was still heavily involved in Afghanistan and Iraq, defence spending was at 4.78% of GDP. It doesn't condemn Germany for doing business with Russian Federation.
The president said US relations with Russian Federation are hurt by "rigged witch-hunts".
As the drama unfolded, a day after Trump launched a virulent public attack on German policy, NATO Secretary General Jens Stoltenberg cleared the room of many officials and the invited leaders of non-members Georgia and Afghanistan so that the other 28 leaders could hold a closed session with the president. But it was Trump who was ultimately brought to heel by the end of the summit.
During the trip, Trump questioned the necessity of the alliance that formed a bulwark against Soviet aggression, tweeting after a day of contentious meetings: "What good is North Atlantic Treaty Organisation if Germany is paying Russian Federation billions of dollars for gas and energy?"
This gives Putin, who invaded the Ukrainian territory of Crimea in early 2014 without major reaction from the West, even more leeway to shape the global balance of power despite its relative economic weakness versus major adversaries like China and the United States.
To put it mildly, the history of heavily militarized European states has not been good for those who appreciate peace.
"I think we could have a very good relationship with President Putin", Trump said, adding that he was not going into the meeting with "high expectations".
Of course, Trump's supporters saw him as the big victor.
On Thursday, Trump had said, "I think they like me a lot in the U.K".
Trudeau, for his part, appeared determined to avoid another summit-closer disaster after last month's G7 meeting in Quebec, where the president excoriated him on Twitter and rescinded his support for the G7 communique upon hearing the prime minister promise to defend Canada against US trade aggression. "The current commitment is to reach two percent by 2024 but with get-out terms that would allow some to stretch it out to 2030". "He left. Short and easy".
"The United States does not support pipelines such as Nord Stream 2 and a multi-line Turk Stream that will only increase reliance on a single source of supply", Perry told reporters.
Referring to an ongoing investigation into ties between his election campaign and Russian Federation, he decried a "rigged witch-hunt" that was hurting the United States and its relationship with Russian Federation.
But what would that mean practically?
Merkel, facing domestic political opposition to pushing defence spending up from 1.2 percent of GDP, said she explained to Trump how much was already being done.
He said the Cold War had ended a long time ago, and that in some areas, such as global security, arms control, and fighting worldwide terrorism, Russian and US interests coincided.
"One thing you need in this alliance is predictability", an unnamed diplomatic official told NBC.