One of Tyler Cowen’s readers asks,
Which avoidable/contingent event in history did the greatest harm? (e.g. the burning of the library of Alexandria)
As Tyler points out, you never know whether avoiding catastrophe X could mean that worse catastrophe Y is in store. But with that caveat, when I think of avoidable events that did harm, here is what comes to mind:
World War One. You can start with the direct, immediate harm, in terms of death and destruction. Then proceed to the post-war flu epidemic, the Communist Revolution in Russia, the collapse of Germany and subsequent rise of Nazism and a second world war; the retreat from globalization and the destabilized world economy, which arguably helped to cause the Great Depression; the fondness that American leaders developed for central planning during the war, which was highly influential in the way that they responded to the Depression–a lot of New Deal initiatives that ratcheted up government were inspired by World War I era government boards.
In summary, before the first world war, global trade was expanding, governments were small, and many people lived in peace. The war unleashed numerous plagues, some of which are still with us a hundred years later.