The paper provides a detailed overview of the existing relationship between the just war theory and international law. It stresses the fact that the two concepts were historically incompatible. The just war theory falls within ethics and appeals to superior principles that were not in accordance with the positivist law theory and the concept of sovereignty upon which public international law was founded. That incompatibility may at first seem as a paradox since the two concepts should be derived from a common base: the idea of justice. Further development of international law has clearly proved that law cannot be separated from the idea of justice and that is, to some extent, closely linked to some elements of natural law. The author concludes that in the domain of the use of force contemporary international law provides a legal frame, which is in accordance with the precepts of the just war theory.