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Meccan Imamate
Over the course of the previous few years, both the Caliphate and the Imamate had occasionally sought to regain the initiative in spreading their faith by conquering neighboring regions (the Muslim ardor ebbed greatly when the conquests slowed to a halt) but all attempts to acquire territory, tribute and followers failed be they directed against the Jews of the south, the Magyars of the southeast, the apostate heretic Maslamahan Hanifites of the east, the Lakhmids to the northeast and the Ghassanid Dynasty in Syria to the north.
Indeed, any attempt to marshal the limited resources of either the Imamate or Caliphate against infidels would only lead to the other Muslim state to invade. Thus, for years, a lingering standoff emerged, each fearing to act lest they leave their respective capitals of Medina and Mecca vulnerable.
As the two Islamic sects diverged over esoteric points such as the use of icons for historical (but not holy) figures, the phrasing of certain stories of the Qur'an and even the exact dates over when the Prophet received given revelations, the two nations solidified their mutual antagonism. Fearing defeat, each Muslim state proceeded to make alliances with assorted infidels. To secure their southern flank, the Imamate would agree to a far-reaching peace treaty and trade agreement with Israel (Yemen). After several defeats to the Ghassanids, the Caliphate would make peace with their northern neighbors.
Neither the Imamate nor the Caliphate could stomach anything other than a non-aggression pact with the Maslamahan Hanifites to the east. Given that the apostates had allied with the Lakhmids (and theoretically their allies Persia, which also nominally counted Magyar-dominated Oman and Israel as allies and client states), no military leaders of note would recommend launching an invasion in ANY direction. Not only was the chance of success against the inifidels low...but the other Muslim state would almost certainly take the opportunity to invade...and probably win.
Hispania
Heraclius II of Hispania, seeking to protect his diplomatic flanks, would arrange the marriage of his son to the granddaughter of Theodore I of Africa. Second cousins, the part were far enough apart that no one could complain about incest (both Heraclius and the heir to Africa had married local nobles in order to guarantee regional support).
It was a large step between the two "Carthaginian" Catholic nations, one which promised to reduce the tension after Theodore usurped his elder brother Heraclius I's stronger claim to the African throne and bring about an alliance intended to maximize the power of the Carthaginian Church.
Kent
Eorcenberht of Kent, though he gained southern Anglia by way of his alliance with the Celts, would look at the map and wonder if he'd made a devil's bargain. As the last major Saxon Kingdom in Britain, and one supporting the Byzantine Church no less, his throne seemed very, very insecure.
Seeing the slaughter of the Saxon clergy and nobility in Northumberland, Mercia and Kent over the years led some to dismiss Eorcenberht as a traitor to his faith. But the Saxon King hardly cared overly much about petty religious dogma, especially as the main difference between Byzantine and Carthaginian Catholic faiths was....who was in charge of the Church, one Pope or the other.
By now certain that the Carthaginian Celts planned his own Kingdom's conquest next, the King would determine that maintaining his Dynasty required a daring move. He pronounced that the Kingdom of Kent would now follow the Carthaginian Church. In this, he hopes for peace were dashed not by the Celts but by his own Saxon nobles, many of whom were horrified by the decision.
Over the past decades, the Kentish Kings had led the suppression of the Pagan Britons of their realm, making Kent (and perhaps East Anglia) the most Christian Kingdoms in Britain. However, the Saxons were not particularly beloved by the local Britons who were already agitating for closer ties to other native peoples, including their religion (not that the peasants cared anymore for Church doctrine than the King).
Dozens of Saxon nobles would rise up in rebellion, including members of Eorcenberht's family. A vicious civil war ensued bringing misery to southern Britain.
Britain
The Saxons were hardly the only people warring among themselves in the Isle. Having been met with great success conquering various Saxons Kingdoms in the southeast, the Kingdoms of Northumberland, Mercia-Anglia, Powys and Gwynedd fell out over dynastic issues and commenced warring among themselves.