There probably would have been no war at all if Germany hadn't given its weaker friend Austria carte-blanche to crush Serbia in 1914. In fact the Serbs, desperate to avoid war, acquiesced to virtually every condition of an lengthy Austrian ultimatum, but Austria invaded anyway.

Berlin had been planning an aggressive war for years. German philosophers held that wars earned respect and made nations strong.

Serbia, Russia, Belgium, and France fought because they'd been invaded. Belgium collapsed, France nearly did, and Russia eventually sued for peace.

The British fought to uphold a decades-old treaty with Germany to maintain Belgian neutrality. It also fought to prevent German military domination of Europe and the English Channel, and to protect British colonies from German annexation. Many, many people believed, regardless of politics, that British national honor alone required the defense of Belgium and France.

It didn't help Berlin when it derided the Belgian treaty as "a mere piece of paper."

The United States entered the war in 1917 when Germany had resumed the unrestricted sinking of neutral ships and secretly promised Mexico the return of the US Southwest in the peace settlement if it would invade the USA. In 1916, German saboteurs had also blown up a major munitions depot near New York City.

If the Allies didn't exactly fight an idealistic "war to end war," they did fight to uphold the force of international law (such as it was), protect existing borders and governments, punish naked aggression in Europe, maintain freedom of the seas, discourage the German and Austrian militarism that led to the war in the first place, and expel their armies from Serbia, Belgium, Luxemburg (which had no power to resist), Russia, and France.

The Allied victory accomplished all of these things, though at horrific cost.

If the peace lasted only twenty years, the lion's share of the blame lay elsewhere.