“War” is a recurrent event throughout history, and the human history is a record of alternations between war and peace. The occurrence of war always brings abysmal pains to its victims and makes impact on the post-war political and economic development; therefore, the study of war is necessary. In this paper, we try to look at the causes of war and its preventions from the realistic perspective. It is the belief of realism that inter-state politics is anarchic, as all the sates are actors driven by self-interest, for whom conflict is an inevitable means to self-protection. In anarchy, states can only resort to the balance of power to stop the invasive ambitions of other states in order to prevent the occurrence of war, which is usually the result of the imbalance of power. But another sect of realists, the theorists of hegemonic stability, argues that the assurance of inter-state peace relies on the existence of hegemonic states rather than the balance of power. No matter for the balance of power or the hegemonic stability, realism overly stresses the pursuit and manipulation of power, which usually leads to the undue extension and of power and results in conflicts and invasions. Therefore, to prevent war from happening, realism has to be employed along with liberalism and constructivism.