> I don’t really understand how and why war happens. Like, it starts out as two parties both wanting the same resource, and then somebody says “over my dead body” or something and then a million other people decide to also join in this fight? I do not get it.
Different wars start for different reasons.
The Great War started because the Serbian government financed a small group of separatists in Austria-Hungary, who assassinated the heir to the Austro-Hungarian throne. The Austro-Hungarian government naturally objected, and presented the Serbians with an ultimatum essentially demanding the ability to investigate & prosecute anyone involved. Serbia refused, supported by its ally Russia, who mobilised & threatened to invade Austria-Hungary & Germany if either did anything to Serbia.
Due to the military realities of the time, if one country mobilised and another didn't, the former country would be able to take great swathes of territory from the latter before the latter could do anything. So of course with Russia mobilised Germany had to mobilise. And of course with Germany mobilised France had to mobilise too. France, meanwhile, really wanted to take back a piece of territory that Germany had won in 1873 (and France had previously won in 1648 …), so planned to invade Germany as soon as Germany & France's ally Russia were fighting.
The Russian army was large but poor, while France's was smaller but deadly. Faced with a two-front war, the German plan was to hold Russia at bay while defeating France first, then swing to the east and defeat Russia second. However, a fight across the heavily-fortified Franco-German border would have been far too slow to defeat France in a timely fashion, so the Germans planned to go around the French border fortresses by marching through Belgium, never mind that Belgium had not agreed to this plan.
So, Germany invades Belgium, invades France, almost makes it to Paris, gets pushed back, the Brits join the fight, the Western Front solidifies into a four-year stalemate in which roughly 3 million men die and 8 million are wounded; Austria-Hungary invades Serbia but does remarkably poorly; Russia invades Germany & Austria-Hungary; Turkey joins the fray, hoping to reconquer some of the Balkan countries which had won their independence.
Ultimately the victorious Allies assign guilt for the war to Germany, imposing reparations which set the stage for the much nastier Second World War; the monarchs of Serbia, Austria-Hungary, Germany, Russia & Turkey lose their thrones and/or lives; Russia falls to Bolshevik Communism (which sets the stage for the much nastier Second World War); Eastern Europe is broken into a patchwork of states with an unstable ethnic mixture (which sets the stage for the much nastier Second World War); millions die; and the Belle Époque, the highest point of western civilisation, gives way to the horror & madness of the twentieth century, the single bloodiest hundred years mankind's ever seen.
As for the assassin who started it all? Too young to be executed, he wastes away & dies in prison of tuberculosis.