What was considered Alexander the Great's greatest victory? Why was it considered so impressive? Which army did he defeat?

The Battle of Gaugamela

Explanation:

This battle was a huge victory for Alexander the Great. It made sure he had completely conquered Persia. It's considered impressive because he defeated one of the worlds, if not the worlds, biggest power at the time. He defeated the Persian Army.

Alexander the Great's conquest for the Persian Empire was one of his biggest successes. He started by taking control of the empire's territories without any defeats. Over the next 8 years, he led his army onward another 11.000 miles and in the end, illness put a stop to Alexander the Great's reign.