Jeanne's father had assimilated to America in his embracing of what it meant to be American.
Prior to his arrest, Jeanne's father had assimilated into American life. He took the American name of "George" over his native Japanese, Ko. His changing of name reflects a desire to immerse himself in American culture. He also represented the American Dream. He starts off as a farmer in California, and then becomes a fisherman with a profitable business that provides for his family.
Upon hearing about the Pearl Harbor attacks, Ko burns his Japanese flag. Even though it was an heirloom, he severs connection with his past in order to demonstrate his allegiance to America. It is at this point where his assimilation is most evident. Part of the reason he is so emotionally broken after his internment is because he merged so much of his own identity into America. He cannot reconcile his brutal treatment with the land that he loves so much. In many regards, his plight is one of the saddest because of his high level of American integration. As a result of the treatment he experiences, he feels betrayed. Ko's inability to fully move on from the horrors of detainment is a reminder of the bond he had with America. It was an absorbing connection that, when broken, left a permanent scar.