system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why was the Hammer and Anvil tactic developed by Alexander The Great so effective?

How was it able to completely destroy the Persian army in the Battle of Issus when Alexander's forces were completely outnumbered 5 to 1?
gpt
In regards to the Battle of Issus specifically, there's a rather easy answer to why it was so effective. 

By the time Alexander led his Calvary (the hammer) into the back of the Persian lines, he had already sent the opposing commander Darius fleeing for his life. He was actually trying to chase him down, when he saw his center was in trouble, wheeled his Calvary back towards the battle, and hit the Persian center from the rear.

Also, when he crashed into the Persian center, he was actually attacking a majority of the Greek Mercenaries who had been placed in the center before battle commenced, not just the Persian infantry. Once the mercenaries (and Persians) realized that Darius was fleeing and the battle was lost, they all began to retreat themselves.

Also of note is that historians have revised numbers showing that the Persians had a 2.5 to 1 advantage. Still significant, but not as great as originally thought.

TL;DR version: It worked because Darius had already fled the field, and the Persians and mercenaries realized this about the same time that Alexander unexpectedly hit them in the rear with his Calvary.