This week, the 29 member states of the NATO alliance that has remained a pillar of collective security in the West since 1949 will gather in Brussels for a crucial high-level summit to decide on the direction of the organisation.
Fifty-five delegations including 29 heads of state or government will descend on Brussels to inaugurate the alliance’s new $1.45 billion headquarters that sits on the outskirts of the Belgian capital.
But the summit is sure to be defined by other matters at what is a critical juncture in the group’s history.
Other than Mr Trump's key concerns about which members pay their bill, here are the key issues that will shape developments at the NATO Summit and the future direction of the military alliance:
The defining photo of the recent G7 Summit in Canada summed up Mr Trump’s relationship with Washington’s allies. Mr Trump was pictured sitting cross-armed and indifferent, as G7 leaders leaned over him as their advisers watch on.
He became embroiled in a Twitter spat with Justin Trudeau over trade, faced criticism from French President Emmanuel Macron and left without signing a joint communique.
Mr Trump now seeks lower EU tariffs on American goods, appears to want allies to buy more US hardware, and does not believe that such multilateral bodies help the US, but rather take from it.
So the NATO Summit has the potential to be as disastrous, if not more so, than the G7 gathering that ripped at the seams of America’s historic partnerships.
Major European states now fear another major split in the Western alliance and a major reduction to the US role within NATO. So if they are able to project an image of stability, and keep Mr Trump’s bull-in-a-china-shop antics to a minimum, the summit could actually be a success.
Read more from Jack Moore at the NATO summit:
Russia is a major concern for NATO, particularly the Baltic states that lie close to the “bear”. Worries over Moscow’s capture of Crimea and its potential to do the same elsewhere in Europe, for example, Estonia, are high.
But Mr Trump has set up a bilateral meeting with Russian President Vladimir Putin in Helsinki just four days after the summit ends. His bid for friendly relations with Russia, while helping to build-up troops in eastern Europe on Russia’s western flank, has sent mixed messages. His Russia policy will be clearer after the two summits.
But elsewhere in NATO, there are calls for stronger measures of deterrence against the Russian threat. German Chancellor Angela Merkel said the Crimea annexation and Russia’s involvement in Ukraine had made it essential “to focus more on defending the alliance”.
“To do that we [must] make necessary arrangements, for example through a presence in central and Eastern European countries,” she said. Russia has long-railed against any NATO presence on its border, calling it inflammatory.
But with Mr Trump appearing to seek to undermine the alliance, other NATO leaders are expected to rally together to form a united front against Russian aggression. But Mr Trump, whose campaign team is under investigation for suspected collusion with the Russian government, will likely not be at the centre of that alliance within an alliance.
This is one of the most unifying issues in the alliance. Members are in agreement that terrorism must be combatted in all its forms. But Mr Trump has previously accused the alliance of not “taking care of terror”.
The only time that the alliance has activated its Article 5 clause, the principle that an attack against one member is one on all members, was the day after the September 11 2001 attacks on the Twin Towers in New York.
In the name of collective security, the alliance committed thousands of non-US troops to the campaign in Afghanistan that ousted the Taliban from power. More than 1,000 lost their lives. Four members, Britain, Canada, Denmark and Estonia, suffered higher casualty rates than the US.
So while NATO allies came to the aid of Washington, Mr Trump has made it clear that members must now do more if they want US protection in return. He has stated that Europe gains more from US contributions and the US is “far away”.
The last time the alliance met, in Warsaw in July 2016, Europe was in the midst of a wave of militant attacks carried out by cells or individuals supportive of Al Qaeda or ISIS, the deadliest being the November 2015 Paris attacks.
While ISIS has been defeated militarily in Iraq and Syria, the threat of terror attacks on the soil of NATO states remains. Manchester, London and Barcelona all suffered deadly bomb and vehicle attacks last year.
Mr Trump himself has pledged to wipe out terrorism from the Western world. So expect him and other NATO members to rally around this issue.
The body has grown over the years from just 12 nations to the current 29. This year, fresh off a recent agreement regarding its official name, Macedonia is confident that they'll get the nod to begin membership talks.
"We have high hopes and I think everything is in the pipeline and it should happen this week," Nikola Dimitrov told Reuters on the sidelines of a Western Balkans summit in London on Monday. "We should be ready to start accession talks."
last month, NATO Secretary General Jens Stoltenberg said the alliance was likely to approve membership talks at the upcoming summit. Mr Stoltenberg also said that the recent agreement with Greece to change the country's name to the Republic of North Macedonia and end a decades-old dispute was "a historic agreement which provides a historic opportunity" for Skopje to join NATO.