People tend to regard the Second World War as the most horrific moment in Human History.
But there was another, that was arguably as dreadful (if not more!), that turned the European landscapes into lunar ones, that killed almost 9 million soldiers and 6,6 million civilians, that shocked the entire world for decades and made humanity lose its confidence in "good science" (for it was the very first technological war), and without which there would not have been any Second World War: World War One.
The website First World War.com provides a complete tour of this four years' war, the Great War: whatever you want to learn about it, there is certainly an answer on this page. Of course there is a chronology of the events and explanations on the reasons, but First World War.com also features many interesting, shocking or thrilling articles. Find out how Hitler experienced the war, and why it is a crucial cause of his later successes and doings; by the way, what if Germany had won? Or perhaps you would like to learn more about events in Belgium.
If you can stand it, what about reading on some of the most dangerous or traumatical weapons of war: machine guns, that mowed down entire waves of attackers and thus played a role in the entrenchment; flamethrowers, that spread deadly sheets of flame and terror among soldiers; and of course poison gas, another means of pressure and a highly technological weapon.
WWI was the very first war during which weapons killed far more than disease. You may want to learn more about the casualties...
And compare what you know of the war with the irony of propaganda.
Or if you don't feel like making it through all this horror, you can read an article on the Christmas Truce: in 1914, soldiers of enemy nations became friends for one night, coming to the no man's land, singing Christmas songs together,... This event, which occurred only once throughout the war and is depicted in the 2005 movie "Joyeux Noël/Merry Christmas", can be considered THE one moment of sanity in all this human madness.
A very interesting and complete website indeed, and an exhortation to remember, in order not to commit again.
Let us learn from History and not repeat it.