Wars are always fought for reasons. Although nominally the fighting was over religious issues, the Reformation being in full swing, the reasons the various phases of the Thirty Years War (1614-1648) were fought over the political and economic control of central Europe. The Holy Roman Empire (the last vestige of the ancient Roman Empire in the West) had become mostly Lutheran under the terms of the Peace of Augsburg (1555) and Germanic princes began to force whatever religious convictions they had upon their subjects. The official Catholic government came at odds to the various fifes that were absorbing former Catholic lands. The Thirty Years War thus started out as a German Civil War, and spread to include the dominant powers in Europe at the time. There were 5 phases to the war (see link) and strangely, 300 years later, Germany was also the central player in a series of wars from 1914 to 1945.
I do not know if the Thirty Years' War was the most outstanding example of a meaningless conflict, but it certainly was a good example of a war that really did not accomplish anything that was anywhere near to proportional to the problems it caused.
The main reason you can say this is because the war devastated many parts of Germany and because it bankrupted many of the countries that participated in it. Some historians argue that Germany's population, for example, dropped from 21 to 13 million people during this war.
In addition, the war did not result in any major gains for any of the countries that fought in it. There were som territorial adjustments, and all German states ended up free to determine their own religion, but that was about it.