A society with an ethical code based on individual rights is absolutely opposed to war.
Individuals who are free to produce, have no interest in war. They have an incentive for economics and peace.
It is impossible for individuals to initiate war. Only a government can do that.
A state that enforces a national mandate for military service is guilty of an abuse of individual rights and should be considered unethical.
A political system that uses force against it's own citizens can easily justify force against another state.
Any state threatened by war should respond without hesitation and if necessary, with a well trained and voluntary military force.
The United Nations began in October 1945 with the creation of the United Nations Charter which states (in Article 1), that it's purpose is:
"To maintain international peace and security and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice....
... and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace".