Japanese complicity in the death of Marshal Chang Tso-lin on June 4, 1928, by an explosion under his railway car on the Peiplng-Mukden Railway, has long been taken for granted. In the absence of a full explanation of what happened near Mukden as well as what happened, in consequence, in political circles in Japan, myth and conjecture have obscured the real significance of the episode. An examination of the record makes it possible to show that it was not another manifestation of the positive policy of the Tanaka cabinet but that it was the first successful test of strength between the radical wing centered in young officers in the Kwantung Army, with civilian sympathizers in Japan, and the more responsible elements of the Japanese government, both civilian and military. Strong disciplinary action by the latter group at this point, perhaps, could have checked a chain of events that eventually led to the ascendency to power of the radical faction and the realization of some of its foreign and domestic programs. The failure to punish by established disciplinary procedures may well have been a fateful decision in Japanese history.