World War I officially began on July 28, 1914, when Austria-Hungary declared war on Serbia following the assassination of Archduke Franz Ferdinand. The event set off a series of alliances and escalations that drew multiple countries into the conflict, ultimately involving many of the world's great powers. The war was characterized by trench warfare and saw unprecedented levels of destruction and loss of life. It was a defining moment in world history, leading to significant geopolitical changes and setting the stage for future conflicts, including World War II.