War is not always evil. Ultimately, war is nothing but change.
Humans have conflicts with each other, not merely because of selfishness, but because of scarcity, error and miscommunication. Normally, the system by which these conflicts are resolved peacefully is the law, whether national or international. However, if the law is unjust, then conflicts are not successfully resolved and efforts are made to change the law. Many systems exist for peacefully changing the law, such as democracy for national laws, and diplomacy for international law and treaties. However, if these systems fail or prove inadequate, the law or political system is changed by war, whether civil, international or terrorist. The purpose of war, therefore, is to correct laws and political systems that one or more parties believe to be unjust. Ultimately, injustice, perceived or actual, is the root of war. The object of war is new law and a new political system which is more just than the last, and therefore a more perfect peace.
The close connection between war and law is self-evident. The bedrock of civilisation is the threat of force and violent death. In a civilised country, if one does not obey the law, more and more force is applied until he does or is killed, otherwise it ceases to be civilised. If the police cannot stop a criminal or gang, the STAR team is called. If they are beaten, commandoes will come. If they are defeated, the army will be mobilized, then the airforce and navy. Ultimately, the force of the law is the force of the gun. The law derives its power from its authority and threat to kill, and from the law emerges peace. Ultimately, peace between men depends on the existence of the law to mediate conflicts peacefully, and the law is upheld by the threat of war. Peace within a nation is therefore dependant on the threat of civil war, and civil war is the guarantor of civil peace.
In the same way, peace between nations is maintained by a balance of power. This is nothing more than a fancy way of saying that they threaten each other with force and violent death. International law and cooperation is built upon a pyramid of deterrents, with the ultimate deterrent being war. If a country does not fulfill its treaty obligations to lower tariffs, it will be sanctioned, then embargoed. If it tries to force the embargo, coast guard, then naval ships will enforce the embargo. If it tries to sink the naval ships enforcing the embargo, war erupts. Ultimately, even the benign trade treaties between nations have devastating military force behind them. International cooperation therefore is guaranteed by the threat of war, and war is the spectre whose threat ensures peace.