World War one
World War One was especially gruesome due to the technique of warfare. Millions died on the battlefields and thus a generation was lost.
Soldiers had to wait for days in a dirty muddy trench waiting for the orders of attack to come in. Then they had to walk om the open field towards the trench of the enemy who were shooting them like sitting ducks. This repeated itself again again and again. You can imagine how these soldiers must have felt.
Meanwhile at home the local governments of the fighting countries tried to keep moral high by putting up propaganda pictures and keeping all the news of the atrocities out of the papers. Unfortunately they succeeded and people stayed enthousiastic to fight in this war.
Was it all worth it? My guess is if this really had been the war to end all wars than maybe it would have been worth it, but history has shown us that many wars followed World War One and I am afraid many are yet to come.
If you want to know more about world war one just go the internet and you'll find hundreds of sites explaining every detail of the Great War.