1895, he left German nationality to escape military service, because Germans have the obligation to perform military service, and he hates war. 19 18 years, he has returned to Germany, so he joined German nationality again, so the Nobel Prize in 192 1 year belongs to Germany. Although he has dual nationality, he is obviously German.
1940, he couldn't get rid of German citizenship, because the Nazis no longer regarded Jews as Germans, and he was in the United States at that time, so he joined American citizenship.
Throughout his love life, he never paid attention to his nationality, nor did he really belong to any country, so it is inappropriate to say that he belongs to any country. Relatively speaking, Germany, as his motherland, is more confident to say that he is German, but most books don't mention who he is, saying that he is a citizen of the world.