So, long long time ago, there was a big fight in Japan. It was between two groups of people- the emperor's army and a bunch of rebels. The rebels had a castle, but the emperor's army wanted to take it over.
Now, the emperor's army was really strong and had lots of people, but the rebels were very smart and sneaky. They had a secret plan to make the emperor's army think that they were giving up and leaving the castle.
The emperor's army believed the rebels' trick and started to leave, thinking that they had won. But the rebels still had a secret weapon- a ninja named Amakasu.
Amakasu and the rebels waited until the emperor's army was really far away, and then they attacked! They caught the emperor's army off guard, and were able to beat them and keep their castle safe.
The Amakasu Incident is the name of this big fight that happened. It's an important part of Japanese history because it showed how clever and strategic people could beat a big, strong army.