A change of allegiance
Khalid came from the Makhzum clan of Quraysh. He initially fought against the Muslims, including at Uhud, before accepting Islam and entering Muhammad’s ﷺ service.
Source: TDV Encyclopaedia of Islam — Khalid ibn al-Walid
Campaigns in Syria
Khalid moved from Iraq to the Syrian front and played a major role in the fighting there, including Yarmuk. Umar subsequently removed him from overall command. His military career is better documented than popular claims of an exact undefeated battle total.
Source: TDV Encyclopaedia of Islam — Khalid ibn al-Walid