Americans, please try to understand that most Europeans don't see a direct connection between the World War II and US policy today. From their standpoint, talk about gratitude is a non-sequitur, at best. To many, it seems like a demand for slavish obedience to US whims. Even those who are genuinely and deeply grateful to the United States get their hackles up when gratitude is obligatory and seems to be equated with obedience.
A more insidious and destructive consequence is that introducing "gratitude" into a discussion comes across as changing the topic, because they don't see the connection in the first place. It leaves the impression that you've given up on arguing the merits and have been reduced to calling in old favors to get your way. It ends up seeming petulant, manipulative and insulting. In short, it polarizes the discussion and eliminates any chance of finding common ground.
That doesn't make it wrong, just ... unhelpful.
Europeans, please try to understand that most Americans do see a direct connection between World War II and the present day. Americans travelled halfway around the globe, to fight and die in Europe's war, and they paid the butchers bill for Europe's failure to act before crisis grew into tragedy. And they stayed -- to nurse weary Europe back to health, at first, and later to defend their allies both old and new against the danger that arose in the east.
Now America is under attack and some of those "allies" won't even stand clear and allow her to defend herself, conspiring to deny America the use of her own armies - the very armies that have stood guard over Europe for more than half a century. That is the thread that binds World War II to the present for Americans, and the reason that they talk about gratitude. When you deny that link from yesterday's tyrants to the present, you seem - to American ears - to be disregarding and dishonoring more than fifty years of steadfast support from America. You seem ... ungrateful.
One side won't win over the other on the question of gratitude, but gratitude itself is a side issue. Let's focus the discussion on the real issues and avoid diving into this emotional and divisive rathole. Hopefully, a better understanding of what the other is thinking will make it easier to take a deep breath, take a step back, and turn the discussion to more productive topics.