That monarchs and States, at least those that consider themselves civilized, have for centuries recognized that aggressive wars and invasions violated the law of nations is evident from the fact that invariably he who started his troops on the march or his fleets over the seas to wage war has endeavored to explain and justify the act by asserting that there was no desire or intent to infringe upon the lawful rights of the attacked nations or to engage in cold-blooded conquest, but on the contrary that the hostile acts became necessary because of the enemy's disregard of its obligations; that it had violated treaties; that it held provinces or cities which in fact belonged to the attacker; or that it had mistreated or discriminated against his peaceful citizens.