The underlying Reasons
The Byzantine power, which was considered the greatest military force on earth at that time, showed an unjustifiable opposition towards Muslims. As we have already mentioned, their opposition started at killing the ambassador of the Messenger of All‚h [pbuh], Al-Harith bin ĎUmair Al-Azdi, by Sharhabeel bin ĎAmr Al-Ghassani. The ambassador was then carrying a message from the Prophet [pbuh] to the ruler of Busra. We have also stated that the Prophet consequently dispatched a brigade under the command of Zaid bin Haritha, who had a fierce fight against the Byzantines at Muítah. Although Muslim forces could not have revenge on those haughty overproud tyrants, the confrontation itself had a great impression on the Arabs, all over Arabia.
Caesar ó who could neither ignore the great benefit that Muítah Battle had brought to Muslims, nor could he disregard the Arab tribesí expectations of independence, and their hopes of getting free from his influence and reign, nor he could ignore their alliance to the Muslims ó realizing all that, Caesar was aware of the progressive danger threatening his borders, especially Ash-Sham-fronts which were neighbouring Arab lands. So he concluded that demolition of the Muslims power had grown an urgent necessity. This decision of his should, in his opinion, be achieved before the Muslims become too powerful to conquer, and raise troubles and unrest in the adjacent Arab territories.
To meet these exigencies, Caesar mustered a huge army of the Byzantines and pro-Roman Ghassanide tribes to launch a decisive bloody battle against the Muslims.