The Middle Heart by Bette Bao Lord
It is the summer of 1932, the year the Japanese conquered Manchuria, China's northeast region. Three children meet and become best friends for the summer, and blood brothers for a lifetime. They are: Steel Hope, the second son of the House of Li, a once-great clan which survives now on handouts from his grandfather, a merchant who trafficks whith the Japanese; Steel Hope's "bookmate," Mountain Pine, Steel Hope's servant and conscience; and the irrepressible Firecrackers, daughter of the Li's gravekeeper, who masquerades as a boy to take the place of a brother killed years earlier by the Japanese. At the end of the summer a tragic event -- a good deed gone awry -- splits them apart until they are grown.
During the subsequent years of war and cultural upheaval, the destinies of the three friends are realized -- their loyalty to each other tested by the demands of politics and patriotism, and by the question of where honor and obligation lie when confronted by love.