The Battle of Ajnadayn
The Muslims under Khalid ibn al-Walid defeated the massed Byzantines at Ajnadayn in Palestine, in the first great confrontation with the armies of Heraclius, opening the way for the conquest of all of Syria.
When the commanders in Syria appealed to Abu Bakr al-Siddiq (may Allah be pleased with him) against the Byzantine hosts, he ordered Khalid ibn al-Walid to march from Iraq. Khalid crossed the desert of Samawa in a swift, legendary march and took command of the assembled armies.
The two hosts met at Ajnadayn between al-Ramla and Bayt Jibrin. The Byzantines were tens of thousands, yet the Muslims held firm and fought excellently until the Byzantines were routed in a crushing defeat, and a number of the best Companions were martyred.
Ajnadayn was the key to Syria, for it broke the strength of the Byzantines in Palestine. It was followed by Yarmuk, then the conquest of Damascus and Jerusalem. The glad tiding reached Abu Bakr on his deathbed, and it gladdened him.
Note — differing reports on the date: The well-known view places it in Jumada al-Ula 13 AH; some say Jumada al-Akhira. Its day is disputed, and some mention 28 Jumada al-Ula.