When you look back at history and try to complete a list of the greatest military commanders. A big win for a potential candidate would, of course, being undefeated. And when you look at that trait the list is very small, in fact, he is one of three and his name is Khalid Bin Walid.
Khalid was a companion of Prophet Muhammad who seems to be often overlooked, but without him, Islam never would have spread the way it did. Over more than hundreds of battles, Khalid remained undefeated. A feat that no one else can claim.
It becomes an even greater achievement when you factor in that his campaigns were against forces much greater than his own. Against the numerically superior Roman Empire or the Persian Empire. This means that he is generally regarded as one of the finest military generals in history. When you look at his best strategic achievement, the standout campaign would be conquering the Persian Empire and Roman Syria all within just 3 years from 633 to 636.
Most of his military prowess is due to his great leadership abilities, getting Arabia to unite into a single political entity for the first time ever. And his great ability to commit completely to his tactics, also getting his troops to believe in his tactics. He is one of the only military leaders in time that was able to perform a pincer movement against a numerically superior enemy. This was seen in the Battle of Walaja where he performed this maneuver, allowing him to be on the list with Hannibal as the only two military commanders ever to do this.
Many believe that his tactical genius was due to his extreme methods: Khalid believed in the complete destruction of his enemies. It is understood that he had to use extreme tactics because he was always vastly outnumbered, so he had to ensure the enemy forces were killed completely.
And Khalid was no coward, according to historical reports, his body was covered in scars. Wounds he had been inflicted by swords, arrows, and lances after so many campaigns. So many scars that people wondered how he was still alive.