Both in scientific literature and popular mind the Romans are considered e the symbol of aggression, militarism and conquest, but the more thorough analysis shows that many of Roman wars were really defensive or at least began as a war of defence and Th. Mommsen’s idea of “defensive imperialism” has a good deal of sense. The fetial law with its concept of “bellum iustum” stands at the foundation of Roman idea of international relations and was (despite all possible speculations) an important step in the world of undeclared warfare of “civilized (Greeks, Carthaginians) or “uncivilized” (Gauls, Germans and others) nations. Most wars (about 60 of 100) of 5th-3rd centuries BC are depicted in Roman tradition as self-defence, while the period from the Samnite Wars till the time of Julius Caesar becomes the time of the defence of numerous Roman amici and socii. On these principles grows the global doctrine of the defence of “human civilization” against the “barbarian world” and the establishment of world order, based on law and justice. One may consider this picture as an instrument of propaganda, but many of these ideas and declarations were real truth.