As other posters have stated in hindsight no war was worth fighting. World War I would be no exception. In most wars though one country begins the war by being the aggressor and leaves other countries little choice but to defend themselves.
Millions of lives lost, billions in property damage, entire generations wiped out in France and Germany. No way, what a complete and total waste. Much of the world's people in the decade following World War I felt the same as I do, viewing it as an unmitigated catastrophe that ought never be repeated. And what was it fought for? Empire? Pride? By accident? I can't think of a single good reason.
Both have made good points. I think that one's motives are always an important consideration.
I am thinking back to the American Revolution which in my opinion was a war that was worth fighting simply because the British were not treating the American colonists equally as the citizens of Britain. So, the American war of Independence was fought because of unreasonable expectations and heavy handed taxation. Could the situation have been resolved a different way? Probably not, as King George and the Parliament looked at the colonies as a cash cow to be exploited rather than as equal citizens with the same rights.
Moving forward a few years. WWII was fought after WWI because the peace ceded German territory to France as part of the restitution. Maybe WWII could have been avoided if the terms of the peace of WWI were more reasonable. But, up until WWII, wars had always been fought and settled by land acquisitions. WWII was necessary simply on a humanitarian front because of the reprehensible things that Hitler's SS and Nazis were doing to any who did not go along with the Third Reich.
I think that Viet Nam is a police action that wasted many lives and did not accomplish very much other than kill many on both sides. Aside from giving battle experience to soliders in "peacetime" , Viet Nam was not cost effective, did not accomplish the goal of keeping South Viet Nam free from communist rule, and wasted more lives than were "experienced" in battle.
I think Iraq is another Viet Nam. The profit motive is the cause of the war. No one has told the truth about Iraq from the beginning to right now. The truth is simply that the oil is worth fighting for and having control over as everything about American business and industry runs on oil. But, the run-up to the Iraq invasion and deposition of Saddam Hussein was to villify him (he was a bad man) and whip up a war fervor that would make the invasion acceptable to the American people. The jury is still out on Iraq, but I think in time, history will show that this was a war of greed rather than liberation.
Applying current day thinking and philosophies to past events is not as fruitful as understanding the context of the decisions made back in the day. Certainly no one wants to fight a war; however, if you're French, and Germans are invading, what do you do? If you're a large Slavic population, one of many suppressed within a dying empire wanting to establish your own state, but are forbidden to do so, what do you do? At the time, early in the war, combatants thought the fight was worthwhile; why would they fight otherwise? A few years later, they clearly didn't think it was worth fighting, if the stories and poetry from that time are any indication, after they realized the full horrific intensity of "modern" warfare. Sadly, whatever political stresses brought about World War I were not resolved, as Act II started a mere 20 years later. Given that, was World War I a war worth fighting? No, because we had to fight World War II anyway as a result. If World War I had solved anything politically, then it would have been a war worth fighting, if that meant the world could have avoided a much broader, deeper, bloodier, and near-Apocalyptic conflict a generation later.
No war is worth starting, but once a war is thrust upon a country, every war is worth fighting. This holds true for World War I also. The nations who forced the war upon others did not benefit either themselves or others. But once the war was started, defending countries had no option but to fight.