How would I describe the American/European opposition in Tender is the Night?
This is an excellent question to consider. The way in which a number of Fitzgerald's stories are set in Europeans and feature Americans operating in the midst of a very different culture clearly demonstrate that this is a key theme of his work. In many ways, we could consider Fitzgerald to be the literary descendant of Henry James, who writes at great length about the meeting of "new" America with "old" Europe in his novels.
It is important to remember the importance of Americans to the economy of Europe in the period where Fitzgerald based his novels. After the destruction of Word War I, Europe deliberately made efforts to attract wealthy Americans in order to gain income to fuel the rebuilding project. The negative side of this was that the stereotype of wealthy Americans who lacked culture and polish, wnd were loud and selfish was developed.
Fitzgerald himself spent the majority of his adult years in Europe, as an American abroad. Thus he was perfectly positioned to see first hand the impact of such Americans on European culture. This novel explores such a damaging dynamic, showing how, in their efforts to attract money to develop their economy, Europeans had to compromise to a large extent on their culture and identity, moving them closer to America and away from what made them distinct and separate. You might like to consider the character and actions of somebody like Tommy Barban in the novel to see this first-hand.