What brought Sparta and Athens together as allies?

They decided to join together as allies to answer the threat from persian empire.

When the persains became large an powerful, and athens and sparta knew that they needed to become allies to defeat the presains.

At that time, the Persian empire had more than 100 thousand soldiers while athens and sparta collectively managed to gather a little under 20 thousand soldiers.

When the persains became large an powerful, and athens and sparta knew that they needed to become allies to defeat the presains.