This is not intended a put down to you; your observation has been repeated in many quarters, but -
WHY would anyone , in this day and age, go to see the movie knowing nothing about Dunkirk?
Again, I'm sure you are aware of the history, but I have no sympathy - hell I have plenty of resentment for - the historical illiterates who walk out of that theater wondering why they weren't spoon fed the remedial history that they missed while smoking dope and playing video games on their cell phones.
Before I went Saturday night I read up on the battle on line. It made it enjoyable and I completely understood what was going on.
I think that seeing the trailers on tv would lead plenty of people to the theatre to learn about Dunkirk. We knew the history but felt it failed miserably.