I believe the climax of the book would have to be the killing of old Mr. Kurihara in Chapter 14. 

The events leading up to this tragic moment communicate a rising tension, beginning with Mr. Sakane's arrest and his family's forced removal from their Berkeley, California home.  The story is unfolds through the eyes of eleven-year-old Yuki, as she describes the Sakane's relocation first to the comparatively local racetrack at Tanforan, then to the camp in the desert at Topaz, Utah.  Their detention extends through the duration of the war, as the internees work hard to establish a community within the camp, struggling to remain positive in the face of hardship and indignity.  Old Mr. Kuritani's shooting by soldiers guarding the barbed-wire perimeter of Topaz because he did not hear their warning to halt as he searched the ground for interesting stones, serves as the climactic event mirroring an explosion of the tensions that have been building from the beginning of the narrative.

Although conflicts continue, the events following the climax fit well into the pattern of falling action.  Mr. Sakane is reunited with his family, Yuki's brother Ken enlists in the controversial all-Japanese unit in the Army, and the Sakane family leaves Topaz to live under sponsorship in Salt Lake City.