Every Western Civilization student learns that many historians see an unduly harsh Treaty of Versailles after World War I as a major cause of the Second World War. Far fewer are aware of the other main legacy of World War I, which was a botched effort to dismantle the ailing Ottoman Empire, which ruled Greece, Bulgaria, Turkey and essentially all of the Middle East for half a millennium. Greece and Bulgaria won their independence from the Ottoman Empire in the 1800s, prior to World War I.
A large share of all of the significant military conflicts of the 20th and 21st century, other than World War I and World War II, are essentially the post-colonial legacies of decisions made at the end of World War I to divide the Ottoman Empire. Many of the rest are legacies of the end of English colonial rule in South Asia.
While the conflicts are often described as ancient religious and ethnic conflicts, and divorced from their larger context, another way to describe them would be as episodes in a large 90 year old war of Ottoman succession that has still not entirely settled out yet. In this view, World War I was not only a failure, it was an even more dismal failure than is generally recognized.