I know what Rav Avigdor Miller Zatsal says about the American Flag. I may be wrong, but I happen not to agree. While hakaras hatov is important, I think that it is important to remember that we are not Americans. And I may be wrong, but I never heard of any other Gedolim saying that we should put out an American flag.
It is brought down that Moshe Rabeinu was punished for referring to himself as a Mitzri. It is important to remember that we are Jews and not Americans, imho.
“Hashem made American soldiers His agents to protect Jews. Hakaros Hatov is a Jewish attribute.”
Maskim.