Prior to the holocaust, did Shuls in Europe display their countries' respective flags? Do Shuls currently display their countries' flags? When did it start and why?
Rabbi Gedalia Anemer, zt'l, the long-time rav of Young Israel Shomrai Emunah in Silver Spring, Maryland, and founder and Rosh HaYeshiva of the Yeshiva of Greater Washington, refused to allow flags anywhere in his shul's sanctuary. He did not want to leave the impression that people were bowing down to a secular icon when they davened. He studied at the Telshe Yeshiva. I don't know if his opinion is common among its graduates or not.