World War One was, according to the commonly held wisdom, unavoidable. The complex web of alliance and counter-alliance that bound the European powers to each other did make declarations of war on the part of each nation more or less inevitable once an inciting incident occurred.
That incident turned out to be the assassination of Archduke Franz Ferdinand on June 28, 1914. Over the next thirty days, declarations of war started one after another, in two opposed chains of political allies. On one side: Austria-Hungary, Germany and the Ottoman Empire. On the other side, the United Kingdom and its Commonwealth, France, Russia, Italy, Japan, and eventually, the USA as well.
It was the first truly worldwide war, fought in Europe, the Middle East, Africa, Asia and the Atlantic Ocean. World War One lasted for four years and a little under four months. It killed 16.5 million people, the greatest single toll of any conflict to that date, and despite the propaganda of the following years, it did not end wars.