World War I came thanks to several reasons, ranging from modernization to a rapidly changing political climate, but one of the biggest was petty family squabbling.
Europe's three most powerful monarchs — George V of England, Tsar Nicholas II of Russia, and Kaiser Wilhelm II of Germany — were all first cousins, being grandchildren of Queen Victoria. When Victoria died in 1901, relations quickly fell apart, with the three forming cliques like a bunch of high schoolers. While George V and Nicholas II managed to be friends and allies, Wilhelm II allied himself with Austria-Hungary and refused to have good relations with England for a pretty bizarre reason: his mom. Victoria of Germany, the eldest daughter of Queen Victoria, was an overbearing mother, and Wilhelm developed a creepy Oedipus complex that later morphed to outright hatred. When Wilhelm became emperor, his mommy issues led to frosty relations with her home country of England.
It would take just one event for everything to fall apart like a house of cards. In 1914, the Archduke Ferdinand, heir to the throne of Austria-Hungary, was shot by a Serbian national, leaving Austria-Hungary threatening war. Serbia, which was allied with Russia, drew Nicholas II into the conflict. Germany stood by Austria-Hungary, while Russia asked for assistance from Britain and France. Kaiser Wilhelm had communicated with his royal cousins via telegraph before the war, but he now finally found the opportunity to stick it to the rest of the family, particularly Britain. What started as a bunch of arguing cousins led to tens of millions of deaths.