World War I was a big war that began in 1914 and involved many countries around the world. It was caused by a number of things, including a country called Austria-Hungary feeling threatened by a small country called Serbia, some long-standing political issues between countries in Europe, and a failed attempt at restoring balance in Europe.
Austria-Hungary was an empire with many different cultures, and Serbia was a small country that wanted to free some of these cultures and create an independent nation. This made Austria-Hungary feel threatened and angry, so they declared war on Serbia.
At the same time, Europe was home to many different countries, each of which had its own government and wanted to have more influence in the region. This led to political tensions between countries that, when combined with the war between Austria-Hungary and Serbia, made Europe very unstable.
Finally, countries like Britain, France, and Germany wanted to restore balance and peace in Europe, but their attempt failed, leading to the outbreak of war in 1914.