(update: big hello to the Crooks and Liars audience, welcome back)
History teaches us that the War was inevitable.
There was this... understanding across the nations of Europe at the turn of the 20th Century. The empires of the 19th Century had made treaties and agreements to protect their global power by dividing themselves into two armed camps. Revolving around the animosities between France and newly-forged Germany, the other nations and empires - Great Britain, Austria-Hungarian Empire, Russia, newly-forged Italy - chose sides to back in case France and Germany decided to start a thing.
It all came from the belief in a Balance of Power working between the nations/empires of Europe: that no one nation would become stronger or more powerful than the others, requiring nations to gang up against the growing "threat" to ensure war wouldn't happen... Except that, by the 20th Century, various powers wanted war in order to satisfy their needs or avenge some slight.
Or, as Captain Blackadder so rightly put it: "It was bollocks."
The splits had formed over slights and injuries spanning decades: France being humiliated by Prussia/Germany in 1871; Russia being slighted over Austria-Hungary's control over the Balkans; Great Britain threatened by Germany's growing Imperial Navy and open desire for empire-building in places the UK already controlled; Germany's desire to make themselves an economic powerhouse equal to a British Empire the Kaiser Wilhelm II - cousin to the British Royals - so wanted to emulate. Underneath all of this was a budding sense of Nationalism - a tribal impulse of patriotism - merged with various elements of anarchism and economic malaise.
By the early 1900s, all of Europe was a literal powder-keg: each nation building up arsenals and weapons of increasing technological lethality that few of the generals and men in power even comprehended how dangerous war was becoming. While the peace held, it was merely over the fact nobody wanted to be the idiot to start the whole thing blowing up. Nobody wanted the blame once the dust settled...
Except for the ones who didn't care.
For all the politicians and men of power who knew to tread lightly, Europe was also filled with ethnic factions subsumed by the aging Empires affected by the same Nationalist pride. Except that Nationalist pride drove them - especially the Serbians in the Austrian-held Balkans - towards a desire for self-determination, the right to form their own nation outside of imperial dominance. They'd seen nations like Greece gain their independence from the Ottoman Empire - a Middle Eastern empire on the edge of the European boiling pot - and they'd seen Italy and Germany form themselves into true nations out of divided squabbling states. So these smaller states, these ethnic groups, sought their own nations.
This brings us to Serbia.
Serbians had been suffering for centuries, a once proud eastern European culture taken over by the Ottomans in the late 1300s. By the mid-19th Century they were able to fight back to gain some independence from the Ottomans only to suffer new rule under the Austria-Hungarians as part of a Russo-Turkish treaty. While Serbia retained some independence as a nation it still had to answer to the Habsburgs in Vienna, and was blocked from expanding further influence in the Balkan region.
The resentments led to various factions in Serbia plotting for action against the Austrians. Violence and riots were common throughout the region leading up into 1914. When the Archduke Ferdinand of Austria decided on a visit to the region as part of his military duties to the Navy (Austria-Hungary had access to the Mediterranean through their hold of Bosnia), he also planned on visiting Sarajevo for a museum dedication on the date of his wedding anniversary with his beloved wife Sophie.
That was June 28.
Welcome to the anniversary date of the starting point of World War I: the assassination of the Archduke and his wife.
Done as a protest against Austria-Hungarian hegemony, for Serbian nationalism pushing for a Greater Serbia dominating all of the Balkans, it was the excuse the powers back in Vienna needed to stomp down on a Serbian nation they viewed as a threat.
Problem was, Russia had become allies of Serbia by then. Russia's interests in eastern Europe had always been there ever since the birth of their own empire. When Austria-Hungary mobilized for a war on Serbia by July 28, it triggered clauses in treaties Russia had with Serbia and with their Entente with France and UK to mobilize, which triggered Germany's mobilization, which triggered France's...
One trigger unleashed another. At no point did any national leader "man up" and say "wait, this is stupid, this is a fight between Austria-Hungary and Serbia, it doesn't involve us!" Germany didn't need to mobilize against Russia... France didn't need to mobilize versus Germany... Germany didn't need to invade Belgium to preemptively fight France, which gave Great Britain the excuse to jump in... except that there were enough people in power in each of those nations who argued that war was war, that it would be quick and easy with everybody's allies lining up to fight it, and now was the time to pitch in.
Like Blackadder said, bollocks.
Whatever ideals or hope there had been in the 19th Century that humanity as a whole was stepping towards a more evolved, artistic, sensible future died in the muddy trenches of the war fronts. Once started, neither side had little incentive to end it fearing the consequences of national collapse and panic. For four years, the European powers pummeled each other until they had placed serious strains on their manpower and resources.
None of it ended well.
Germany, trying to destroy Russia from within, unleashed a communist uprising in the heart of a frayed Russia that led to the rise of the Soviet Union and to the horrors of Stalin. Austria-Hungary fell apart through a prolonged war that drained their resources. Italy, jumping in late on the side of the French-UK-Russian Entente, found their fortunes ruined in disastrous military campaigns that collapsed their government, leading to the rise of the Fascists under Mussolini. The Ottoman Empire fell apart through British intervention in the tribal uprisings across the Middle East. Great Britain and France lost hundreds of thousands of men against the German lines. Germany used up much of its resources and men as well. Germany's desperation against the UK led to their attacking American shipping and trade interests, dragging the United States into the war.
When the fighting finally stopped on November 11 1918, it was due more to fatigue on all sides than due to any actual victory. But Germany's government fell apart as a result of the armistice, and France and Great Britain wielded enough influence on the following peace process that its lopsided punishments on Germany convinced a good number of politicians and historians to note that the peace wouldn't last (and it was because some of them like General Foch believed the treaty was too lenient and would allow Germany to rebuild). And we all know what that bloody Treaty of Versailles led to...
It didn't help that some of the underlying issues causing the war - the fervent tribalism that masqueraded as nationalism, for example - weren't properly resolved. France and Great Britain still had their empires to maintain after all, and they exerted their influences into the Middle East by carving up the remnants of the Ottoman Empire into nations that forced the wrong ethnic groups in the region into the wrong states. While the bloody history of the Middle East has existed long before the European map-makers made their mark in the region, the drawing up of Syria, Iraq, Iran, Saudi Arabia, Palestine (without an Israel at that time), Turkey and other regional nations with reckless disregard for ignored groups like the Kurds and ignorance of the divisions between Shia and Sunni faiths certainly exacerbated tensions to where we've got the bloody chaos the world endures to this day.
It was 100 years ago the modern world was born. In fire and in blood and in death. We've been dealing with the consequences ever since, more than any other historical event preceding it.
God help us all today.