It is argued in this paper that the phenomenon of forced migration in war constitutes, in itself, a serious violation of international humanitarian law. The agency of government or military command is behind the military or political action that provokes population flight; the short and long-term mortality and morbidity always associated with forced migration occurs disproportionately and indiscriminately to civilian non combatants; and the dissolution of identity, the assault on dignity, the destruction of personal and community records, and the sweeping loss of livelihoods occasioned by war-induced forced migration represent in themselves war crimes or on a grand scale crimes against humanity. This paper presents evidence to substantiate the claim that forced migration in war inflicts intense and extended suffering on civilian populations. Reference is made to Hague and Geneva law, the two international human rights covenants (ICCPR and ICESCR) and to the Refugee Convention to find elements of what should arguably be advanced as the constituent basis for defining forced migration in war as a distinct and independent crime in international criminal law. In much of international humanitarian law, empirically grounded recognition of a new class of grievous injuries or a new category of people to protect leads to an expansion of a preexisting framework (civilian protection) or an entirely new treaty or convention (cluster munitions). The suggestion made here is that forced migration in war be considered within that historical continuum—not as a prevalent and (largely) unavoidable process but as a newly recognised crime.