“Horton Hears a Who” was written as an allegory for Hiroshima and U.S. occupation of Japan. Although Dr. Seuss, wrote only children’s books, he had strong political opinions. He was a liberal democrat and supported FDR’s New Deal. He created political cartoons dealing with the Holocaust and discrimination against African-Americans and Jews. 
For this reason, it is no surprise that the seemingly whimsical “Horton Hears a Who” is also a political commentary. Geisel strongly supported the internment of Japanese American interment during the second World War. After the war, “Horton Hears a Who” was his refutation of his previous opinion. He dedicated the book to a Japanese friend. At one point the line, “A person’s a person, no matter how small!” was used as a pro-life slogan, but Geisel fought against this use of his story. From made-up words to silly rhymes Dr. Seuss entertains children and makes serious political statements. More about Seuss’ political views at the source