Khalid's Last Engagement With the Persians

Khalid secretly left the front lines, went on pilgrimage to Mecca to fulfill his vow to Muhammad. But unbeknownst to him, a new front had opened up. The Persians were so desperate at this point that they made an alliance with, of all people, their old enemies, the Romans. Khalid used the Euphrates to trap their army and demolished them yet again. This is not to say that fighting on this front was over. In fact, the Persians would later counter-attack and retake some of their territory.