The physical and emotional exhaustion resulting from years of trench warfare, including the human toll exacted by the introduction of new means of killing and maiming like machine guns and chemical weapons, combined with the enormous financial costs associated with the war, played major roles in compelling the warring parties to terminate the conflict. All the participants in the fighting -- Britain, France, Germany, Russia, Turkey -- were financially broken as a result of the war. All of those countries experienced enormous losses in a generation of young men wasted for no real reason save the folly of man. The United States, which didn't enter the war until 1917, experienced its own psychological transformation, as the nationalist pride and giddiness about an overseas experience were soon replaced with a more sober perspective on the meaning of war.
As the first world war dragged on, its meaninglessness began to take hold more and more broadly across populations. In Russia, Czar Nicholas II, already weakened by corruption, ineptitude, and a growing revolutionary movement against the Romanov Dynasty, was bled dry by his decision to wage war against the Germans. The Russian population, already emotionally exhausted by domestic political developments, grew extremely tired of its participation in the war. The war's demise would see the end of the great empires, the Austro-Hungarian, Ottoman, Russian, and German -- and the other major colonial powers, Britain and France, would witness the beginning of the end of their global positions.
World War I was the ultimate war of attrition. Its defining characteristic, the series of trenches each side dug to protect its soldiers while slaughtering those of the enemy, became synonymous with the war's futility. Adolf Hitler, a young soldier in the German Army, was famously influenced by his own experiences fighting in the trenches and being wounded by chemical weapons, and swore that Germany would never again pay such a price for the crimes of others (Hitler, of course, blamed Jews first and foremost despite that particular group having had no role in the war's start, duration or finish). Such was the emotional toll endured by Germany's population that the German people would rally around the rising politician's declarations against the terms of the Treaty of Versailles and against ever again finding itself fighting in trenches. The German High Command's development of a strategy utilizing massive numbers of tanks -- Blitzkrieg, or "lightning war" -- was a direct result of its experiences during World War I, and it would prove enormously effective in the opening phase of the war that followed "the war to end all wars."
The populations of Europe had grown thoroughly disillusioned with war by the latter years of this conflict. Its causes and purpose were sufficiently convoluted and vague to elude the comprehension of much of each country's population. Familial ties between monarchies only furthered the notion that these populations were being sacrificed for no greater good.