Conflicts of States can only be decided by war; States being towards each other in a state of nature, their relations are not legal or moral.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

““War has the higher significance that through it the moral health of peoples is preserved in their indifference towards the stabilizing of finite determinations.” Peace is ossification; the Holy Alliance, and Kant’s League for Peace, are mistaken, because a family of States needs an enemy. Conflicts of States can only be decided by war; States being towards each other in a state of nature, their relations are not legal or moral. Their rights have their reality in their particular wills, and the interest of each State is its own highest law. There is no contrast of morals and politics, because States are not subject to ordinary moral laws.” source