If we didn't go to war on terrorism,hitler or north korea (vietnam) thousands of innocent lives thousands to the billions of more innocent people families and children would have been dead rather than us doing so so tell me would you want this to go on in iran if we didn't enter the war on terrorism or are you that heartless?!
There have been a couple of justifiable wars since World War Two. Specifically the war in Afghanistan was a justified war. They were the base of operations of Osama Bin Ladin, and did not help the United States find him. While there may have been a few wars that were not justifiable since WWII, WWII was not the last justifiable war.
In spite of continuous conflict around the globe, few wars are justified because they do not contain the components that make them legitimate. Namely, negotiations were not conducted in an effort to avoid war, the causes for going to war were unjustified or tainted by personal gain, nations engaged in conflicts that were impossible to win and there were heavy civilian casualties. World War II was the last just war fought because it met all the criteria.