593 (POD)- The Roman Emperor Maurice allows his troops to pull back across the Danube from their offensive against the Avars to winter in a frontier fortress.
594- The Roman army again crosses the Danube and inflicts a final, heavy defeat on the Avars before pulling back across the Danube, some staying behind to help guard the frontier while the rest head to Constantinople for a victory parade. Among the dead in that final defeat is an all but unknown officer named Phocas.
595-614- Maurice spends the last years of his reign campaigning against the Slavs that have settled in the Balkans in the Avars' wake and persecuting the Monophysites. The Persian Shahanshah Khosrow II, grateful to Maurice for his assistance in regaining the Persian throne and married to Maurice's daughter Mary, decides to leave Byzantium alone and campaigns against the Turks in Central Asia.
615- Maurice dies and his son Theodosius rises to the Eastern imperial throne as Theodosius III while his other son Tiberius becomes the Western emperor and reigns from Rome, the 'second capital'.
616-633- Business as usual. With Maurice dead, the Eastern Empire (Byzantium) and Persia have the occasional skirmish along the frontier in Armenia and Mesopotamia, but nothing like OTL's devastating war. Theodosius also sends troops to assist his brother Tiberius in Italy, further draining the imperial coffers to little effect. Meanwhile, in Arabia, Muhammad and Islam arrive on schedule.
634- The Muslims begin pouring out of Arabia, invading Palestine and defeating the Imperials at Ajnadain.
636- Arabs defeat Imperials at Yarmuk, near the southern Syrian border.
637- Muslims invade Sassanid Persia, defeating the first Persian force sent against them at Al Qadisiya, at the fringes of Persian territory. Khosrow II gathers his centralized forces and moves south from Ctesiphon against the rapid advance of the Arabs, meeting them in battle at Vologesias. After a hard fought battle, in which the Persians use armored elephants in addition to their heavily armored dehgans and lighter armored horse archers, Khosrow II manages to defeat the Arab force and pursues them out of Mesopotamia and back into the deserts of Arabia.
638- The Muslims take Antioch, bringing all of Palestine and Syria under their rule. The Imperials finally manage to stem the Muslim advance at the Taurus Mountains of Anatolia.
639- Muslim forces invade Egypt and make another try at Persia. The advance into Egypt is swift while, in Persia, Khosrow is more than ready for them this time and inflicts a heavy defeat at Uruk. Correspondence is exchanged between Khosrow and the Muslim Caliph and the Caliph signs a 50 year peace agreement with Sassanid Persia. Khosrow, meanwhile, is happy to encourage the Caliph to continue his advance against the Eastern Roman Empire and, in fact, readies his forces to take advantage of the situation.
640- Imperials defeated by Muslims at Heliopolis on the Nile. Forces used by the Caliph against Persia in OTL start to be diverted to either the Taurus Mountains or to Egypt.
Khosrow II dies and his eldest son rises to the Persian throne. The typical intrigues upon the ascension of a new Shahanshah erupt and Khosrow's son is forced to deal with them.
641- Alexandria falls to Muslim forces in Egypt and Al-Fustat (Cairo) is founded. The Monophysites of Syria, Palestine and Egypt, tired of endless persecution under rule from Constantinople, welcome the Arabs as liberators and begin converting to Islam.
642- Libya falls to the Muslims, the Monophysite populace there also welcoming them as liberators.
643-650- A war is fought between the Eastern Roman Empire, who is already trying to hold back the Muslims at the Taurus Mountains, and Persia. Finally settled on the throne, Khosrow's son invades Roman Mesopotamia and Armenia and sweeps across the regions to the very edge of the Anatolian heartland. There, however, he suffers a defeat at the hands of the Imperials and, after a bit more of back and forth fighting, Theodosius III cedes Roman Mesopotamia and all of Armenia to Persia (roughly equivalent to what the Muslims gained in OTL). Persia is well enough pleased by the territorial gains and settle down to absorb their new conquests and eye the continuing war between the Imperials and the Muslims, eyeing recently conquered Syria and Palestine to give Persia access to the Mediterranean.
How's this?