After the conquest of Mecca in 630 CE, Christian rulers may have been worried because the success of the Muslims in conquering Mecca and the surrounding region could have been seen as a threat to their own power and influence. The conquest of Mecca also marked a significant shift in the balance of power in the region, as the Muslims had now established themselves as a major political and military force.
In response to the conquest of Mecca, some Christian rulers may have taken steps to strengthen their defenses and prepare for the possibility of future attacks by the Muslims. It is also possible that they may have sought allies or formed coalitions with other Christian rulers in order to present a united front against the Muslims.
As for the Muslims, it is likely that they were aware of the potential for attacks by Christian rulers and took steps to prepare for them. According to some accounts, the Muslims formed alliances with tribes and other groups in the region in order to strengthen their defense against potential attacks. They may have also taken steps to improve their military capabilities and to secure key strategic locations in order to defend against potential attacks by Christian rulers.
It is not clear how the Muslims planned to counter a specific attack by the Byzantine Emperor Heraclius, as this information is not available. However, they may have used similar strategies as those described above, such as forming alliances and strengthening their military capabilities, in order to defend against potential attacks from any Christian ruler.