Most non-Americans get it. Many Americans don't identify with any specific part of their ancestry. Maybe they have ancestors from so many different countries and cultures it's all "watered" out with nothing sticking out.
Others haven't been encouraged to be other than "Americans" by their parents (who both might be immigrants themselves). Yet others don't have a clue where their ancestors came from, because they're not interested, and tend to be pretty clueless in general about *anything* outside of the US. Ironically, these are the ones who will get offended when we Europeans call them history-less.
I am sure there are other reasons as well. Somebody who calls hirself Something-American will identify closely to hir Something part, maybe even speak the language, cook the food, celebrate the festivals, or in some other way be part of actual Something culture, or Something culture as it has developed in their part of America. Often they have a close ancestor, like one or both parents, or one or more grandparents, who are from Somethingland, but they can just as well be people who one way or another have managed to maintain a strong Something identity through many many generations. Some Something-Americans even identify with several parts of their heritage, for instance, an American who identifies with two or more cultures or ethnicities will call hirself Something-American or Something-else-American - or just American - according to which culture is in focus.
You forgot the S in descent. Non-americans would understand that you consider yourself Spaniard or at least one of your parents is a Spaniard. Many USAmericans are so brainwashed by newspeak-games and racist fairytales that they think "white" is an ethnicity or that you can "be" Polish, Italian, Greek or whatever without speaking a word of the language.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.