Warfare is human conflict taken to the most violent degree. Human conflict has been and will be with us always. Throughout history, much violent conflict centered on resources. Conflict occurs over many resources such as food, water, and even your mate. One of the things that separate humans from the rest of the animal population is our ability to reason things out. Many times, conflict may be resolved without violence by thinking things through and cooperating. However, even in the human world, this is not always possible.
The issue of slavery in the United States was decided by violence such as this country had never seen before, nor since. We lost more men in that conflict than any other our country has been involved in. It was well worth it. The primary issues of slavery and the federal government over the state government needed to be resolved. To resolve these issues without warfare is almost inconceivable. The issues were too important and impacted the lives of common people far too much. This is one reason why so many good southerners fought for something as evil as slavery. This was a good reason why so many good Germans and Japanese fought for evil during World War II. They were defending their way of life.
I have studied the biographies of thousands of soldiers. The most common reason (Not even close) that they cite as being why they became soldiers in the first place was to defend their way of life. I am referring to the initial decision to join up. This is long before they become comrades in arms and decided to risk their life to protect a fellow soldier. The threat of altering the way of life of just about ANY group will trigger organized violence. In larger groups, this takes the form of warfare. This is human nature. It is as much a part of us as eating and drinking. As much as we would like to, we will never be able to get rid of warfare. Issues will need to be decided that will not have any other means of resolution. In other words, diplomacy has its limits. World War II is a recent example. In addition, the European part of World War II is also an example of trying to hold back the violence until it gets out of control.
If England and France had fought against Germany for Czechoslovakia in 1938 (Like they had promised) the resulting war was projected to have lasted about 6 months instead of 6 years. (As it turned out, the German High Command was planning a coup if war did break out. If that had succeeded, the war would have lasted only days. This was unknown until after the war.) Instead of avoiding the war, backing off ended up making the war far more destructive. As much as we all dislike war, it is a necessary part of life. Certain issues are important to fight for, even if that results in open warfare.