The most often used defense of war is that it is sometimes the only way of stopping evil. But, this claim is self-defeating. Claiming that war is sometimes the only way of stopping evil people from ruling the world also implies that it is only a question of time until evil people take over. Why? Because we can't assume that the good guys always win.
For example, if we assume that war is necessary only 1% of the time, this means that there is one year of necessary war for every 100 years. This also means that there were about 20 years of necessary war since the birth of Christ, and that there will be 20 more in the next 2000 years. That's 40 years of war, or ten four-year wars. How safe is it to assume that all ten of these wars would be won by the good side?
But even if we assume this unlikely outcome, we know that there will always be more necessary wars further in the future. So, if our assumption that war is sometimes necessary to prevent the rule of evil is true, it is statistically inevitable that the evil side eventually wins some of these wars.
If it is true that war is sometimes the only way of fighting evil, then, after the evil side wins, there is no way of persuading that evil side to become good. This means that we would eventually end up with a world ruled by evil.
In Canada, we often hear the following slogan: "They fought so we can be free." Every new generation of school children gets to hear this slogan around Remembrance Day, when Canadians express their gratitude to the their soldiers that fought in the world wars. The fact that the slogan gets transmitted from generation to generation suggests that the argument for fighting in WW1 and WW2 is that if the Germans and their allies won, freedom would have been forever destroyed in all the places they took over.
Fortunately, it is not true that war is ever the only way of fighting evil.
There is an obvious historical example that contradicts this theory. Beginning in 1592, Europeans started invading North America and committing massive genocide of the indigenous population. They were the evil anti-freedom side in this conflict. However, most of us believe that human potential for freedom has not been destroyed in North America, even though the equivalent of the Axis powers conquered this continent.
We may not think that the British invaders of North America were comparable to Nazis, but historical research suggests otherwise:
“Hitler’s concept of concentration camps as well as the practicality of genocide owed much, so he claimed, to his studies of English and United States history. He admired the camps for Boer prisoners in South Africa and for the Indians in the wild west; and often praised to his inner circle the efficiency of America’s extermination – by starvation and uneven combat – of the red savages who could not be tamed by captivity.” -- P. 202, “Adolph Hitler” by John Toland
“over 100 million killed” “[Christopher] Columbus personally murdered half a million Natives” -- American Holocaust by David Stannard
Every time we perform the simplest of tasks, tasks that have minute negative consequences, we consider multiple scenarios, and we pick the scenario that we think is the most favourable or the least painful. But, most people's imagination hits a brick wall when it needs to find more than one possible way of responding to a military intervention. Over eighty million people were killed in the two world wars. I can imagine many scenarios in which there are fewer than eighty million victims. In all of those scenarios, the ideals for which we claim those millions died would still be preserved.