Okay kiddo, so a long long time ago, before cars and tanks were invented, people used to go to war on horses. Horses are very strong and fast animals that can carry people and supplies long distances.
When soldiers rode on horses, they were called cavalry. The soldiers on horses would charge towards the enemy soldiers to try to defeat them. Horses were also used to pull things like wagons full of weapons and other important stuff.
For knights, which were fancy soldiers who had a lot of expensive armor, horses were like their best friends. Knights would ride around on horses with their shiny armor and big swords to try to defeat the enemy.
Unfortunately, horses sometimes got hurt or killed in battle, which was very sad. But they were very brave and helped people fight for their countries a long time ago.