The statement means that, in times of war, people stop caring about laws. Leaders and soldiers often commit acts that society would condemn under normal circumstances. Moreover, civilians might also become less law-abiding, as they are often in desperate situations and the state is unable to persecute those who do commit a crime.
I think the statement is mostly true, as war is a state of emergency that changes how law applies. However, since World War II, the existence of international law and international tribunals have contributed to the development of laws of war, which have made this statement less true than in previous decades.