A long time ago, there were a group of countries that were friends with each other called alliances. These alliances helped them feel safe and protected because if one country was attacked, the others would go help them.
One day, a bad thing happened. A prince from Austria-Hungary was assassinated in a place called Sarajevo. The Austria-Hungarian leaders were very angry and blamed a group of people from a nearby country called Serbia.
They wanted Serbia to say sorry and to stop helping the people who hurt their prince. Serbia didn't want to say sorry because they didn't think they did it.
Austria-Hungary was really mad and decided to declare war on Serbia. Their friends, Germany, said they would help. The friends of Serbia, Russia and France, said they would help Serbia. And so, a lot of countries ended up fighting in a big war called World War I.
Austria-Hungary thought they would win the war quickly, but it ended up lasting a long time and many people died. It was a very sad time for everyone involved.