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But some theoreticians disagree with this perspective. Thus, Byers writes that "when realism replaced liberalism as the theoretical mainstream of International relations in the 1930s and 1940s, international law received little attention as it was widely consider an epiphenomenon to the international struggle for power of self-interested nation-states." [Thomas Diez, Ingvild Bode, Aleksandra Fernandes da Costa, Key Concepts in International Relations, p.110]
In response to that, "empirical reasoning has disproved the realist perspective on several accounts": Franck said that "powerful states did show deference to international law all the while it appeared to be in contrasts to their interest", further Akehurst and Wheeler said that those "states frequently went to considerable lengths in order for their actions to remain within international legal boundaries". [Thomas Diez, Ingvild Bode, Aleksandra Fernandes da Costa, Key Concepts in International Relations, p.112]
All in all, we can't rely on wars to provide improvement of international law because that would mean great atrocities and massive sacrifices. To achieve progress of international law without any more bloodshed, we need to totally reject any possible gains in wars and only concentrate on peaceful resolution of disputes.
This goal is well demonstrated in the UN Charter:
"AND FOR THESE ENDS to practice tolerance and live together in peace with one another as good neighbors, and to unite our strength to maintain international peace and security, and to ensure, by the acceptance of principles and the institution of methods, that armed force shall not be used, save in the common interest, and to employ international machinery for the promotion of the economic and social advancement of all peoples."