Beyond Trump and his criticisms of Germany, the question of Russia is high on the agenda.
While seeking to strengthen EU military might, NATO's chief is urging against isolating Moscow.
NATO Secretary-General Jens Stoltenberg said:
"We don't accept his legal annexation of Crimea, we don`t accept cyber propaganda, interference in domestic political processes.
"But for me that is not an argument in favor of isolating Russia or stop talking to Russia. Actually, I think the opposite. I think when tensions are high it is even more important to talk to Russia."
Moscow will be closely watching what comes out of the Brussels summit.
"Russia is regarding as a threat the forthcoming change of strategy, especially the initiative 4х30 which means to have ready for action 30 ships, 30 battalions and 30 air force squadrons," said said Aleksandr Goltz, a Russian military analyst.
But all of this depends on the amount of money NATO members are willing to stump up - which is of course a hot topic Trump is fiercely pursuing.
Brooks Tigner, from Jane's Defence Weekly, commented: "The key factor here at the summit is for the Europeans to convince Mr Trump they are spending enough. There seems to be a worrying trend in Mr Trump's mind of linking defence spending with trade issues."
Reporting from NATO headquarters, Euronews' Andrei Beketov said: "With Britain leaving the EU, NATO membership could be renegotiated. And, if the World Cup goes well, you could imagine there might be a jovial meeting between Trump and Putin. But with all the military top brass congregating at the new NATO headquarters, the seriousness of this summit is underlined."