Although the 29-member military alliance's annual meetings have traditionally been fairly by-the-books events, expectations are different this year - thanks, in large, to Donald Trump. The U.S.is spending many times more than any other country in order to protect them.
The United States has been the biggest contributor to North Atlantic Treaty Organisation since Western nations founded the alliance in 1949 to help prevent any more world wars and shield Europe from the Soviet Union.
European officials say while USA defence spending makes up 70 percent of combined allied governments' military budgets, just 15 percent of US expenditure is spent in Europe on NATO-related defence.
"It's certainly going to be an interesting time with NATO", Trump said Tuesday morning as he headed to Brussels. Poland's spending, however, was at 1.99% of GDP. That includes running numerous military bases, missile defense systems, training operations, and more.
Trump departs Tuesday on a four-nation tour amid simmering disputes over trade and military spending with fellow Western democracies and speculation about whether he will rebuke or embrace Russian President Vladimir Putin. "It would just be once more a transitory verbal tiff, as opposed to a change in policy, which leads to a true weakening of the alliance", says John Herbst, former USA ambassador to Ukraine, now at the Atlantic Council. Instead, years of defense cuts followed, after the 2008 financial crisis.
NATO, which was founded soon after World War II to counter the influence of the Soviet Union, officially suspended all cooperation with Russian Federation after Putin seized Crimea in 2014.
"I have not, no I have not", he said.
Tusk told Trump to remember that as he sits down with Putin on Monday.
"They kill us on NATO", Trump told the rally in Montana.
"Dear America, appreciate your allies, after all you don't have that many", Tusk said, before reminding Trump that European troops had fought alongside Americans in Afghanistan following the September 11, 2001 attacks on the United States. "'And it means a lot more to you than protecting us because I don't know how much protection we get by protecting you'". The think-tank was referring to the pledge at NATO's 2014 summit meeting that members would "aim to move toward" spending at least 2 per cent of gross domestic product on defence by 2024.
As Trump flew toward Brussels on Tuesday, European Council President Donald Tusk sought to set down brightly colored markers for Trump's arrival. After spending $3.4 billion on that initiative last year, the Trump administration has proposed boosting it to $6.5 billion in the 2019 budget year.
On another front, Trump has connected the defense issue with trade, criticizing Europe for a $150 billion trade surplus (though it can also be calculated as $100 billion) and trade barriers.
German Chancellor Angela Merkel and British Prime Minister Theresa May will join Trump for the two-day meeting, which comes during divisions among America and its allies over policies on trade and pulling out of the Iran nuclear deal.
As with many Trump crusades, there's truth to his claims-but more to the story.
Trump made the remarks in preparation for the Brussels NATO summit this week, where Russian President Vladimir Putin will participate.