By 622, Heraclius was ready to campaign. He led his forces in person, the first ruler to do so since the 390s. He led the army behind the Persians to Armenia, causing Shahr-Baraz to withdraw from Asia Minor, after which the Persians suffered their first defeat. Heraclius returned to Constantinople to make a truce with the Avars, leaving his army in the field, which did not mutiny. In 623 he returned to the offensive, going through Armenia and Azerbaijan to burn the Shah's castle at Ganzak. Leaving a trail of destroyed Sassanian outposts, he almost made it to their capital Ctesiphon before winter. In 625, he moved past Ararat to capture Martyropolis and Amida. In southeastern Anatolia, the two armies met in frontal battle. Though Shahr-Baraz ambushed Byzantine forces inflicting heavy loss, Heraclius' personal courage and advances spurred his reserves to defeat the Persians again. The odyssey continued, however, when Chosroes II ordered a huge conscription and had his army march west, to implement an attack on Constantinople in alliance with the Avars. Heraclius did not come west, but remained to ravage Persian Azerbaijan with new Khazar allies. He left the defense to Sergius and his generals. The Persians got all the way to Nicomedia, and began sending small harassing flotillas to the capital, while the Avars laid active siege to the western walls. Though only possessing a small cavalry garrison of 12,000, Constantinople was ably defended by the walls and its populace, themselves encouraged by Sergius' daily processions with an icon of Mary. When in the first dew days of August Greek vessels destroyed flotillas of Persian and then Avar craft, the two allies panicked, and struck their camps. This marks the beginning of Avar decline. As for the Sassanians, Heraclius renewed the offensive in early 627, marching on the Shah's palace at Dastagird near Ctesiphon. Razates, the new Sassanian commander, avoided battle as long as he could. When they did meet near Nineveh, a daylong battle resulted in Greek victory. Dastagird was burned. Humiliated, Chosroes was murdered by his son Kava Siroe. The new Shah surrendered unconditionally, returning all conquered lands and captives, together with an indemnity and the True Cross. Heraclius entered Constantinople with the Cross in September 628, to the triumphal acclamation of his people, secure in the knowledge that Persia would never re-emerge as a threat.
After an interlude of six peaceful years when Byzantium appeared restored, Heraclius endured a disastrous epilogue to his rule from 634-38. In 622, the Arabian Muhammad had begun to preach Islam. By his death in 632, his new religion had gained a foothold in Arabia. In 634, as an extension of his successors' efforts to prevent decomposition of what was really a new super-tribe, Arab Muslim forces expanded into Byzantine Syria. A small Byzantine garrison was destroyed, and Heraclius, in Palestine to return the True Cross to Jerusalem, gathered an army of 80,000 men to meet the new threat near Antioch. The Muslim forces then fell back, awed by numbers, and the two armies met again on the Yarmuk river south of the Galilee in August. When a sandstorm emerged from the south and disoriented the Byzantines, the Muslim general Khalid charged. Byzantines gave way, and were massacred. By 637, Jerusalem had surrendered along with all of Syria, while Muslim armies were moving into Iraq and Egypt. Sickened physically and emotionally, Heraclius retired in 638, dying three years later.
As a footnote, Heraclius too was unable to escape theological problems with the West. In 634, his Patriarch Sergius had articulated a new formula to please Monophysites and Orthodox. It was that while Christ had two natures, they possessed a single motive force, or energy. It was still controversial. In particular, the monk Sophronius condemned it, and when he became Patriarch of Jerusalem, his opposition may have eased Muslim entry. Finally, in 638 Sergius made another pronouncement, again under Heraclius' orders. Monotheletism, it was that Christ was of two natures, and did not have a single energy, but possessed a single will. All Eastern Patriarchs agreed, but now the Western Pope John IV condemned it in 641, shortly after Arab armies arrived in Alexandria Egypt.
The century from 550-650 in the East could be called the 'hundred years disaster', 'the fall of the Dark Ages curtain', or, most charitably, the transition from Eastern Rome to the Byzantine Empire. Symbolic of this, by Heralius' death, the language of law, administration, and government was Greek, which had already become the language of art and intellectualism. Beyond this, though, the essential components of the Byzantine system were in place: separation from the West; Barbarians raids and settlement south of the Danube, even in states; and Islam as an ever-present threat in the Middle East and Anatolia. Out of this crucible emerged a state and society with a new political glue.
Mentioned earlier was the third wave of Barbarian invasions, consisting of Avars and Slavs. In terms of demographic and ecological effects, their impact is similar to that of the Lombards in Italy, such that they should be considered part of the same grouping. By the time of Phocas, there was large-scale Slav settlement beneath the Danube, as far south as Moesia and Thrace in Central Greece. It was a demographic and cultural disaster for the region. During the Slavic settlement, Greeks became frantic, large numbers of them fleeing for the coast. The great majority of old towns and settlements ceased to exist, with traditional Roman culture all but disappearing. Like Italy, the terrace system decayed as well, opening the way for youngerfill erosion and the contraction of agricultural yields. By the 610s, Byzantine possessions here had been reduced to coastal strips, and just like in Lombard Italy, the Dark Ages of irreversible natural and human decline had set in. As well, a slavicization of Byzantium would allow later Bulgar encroachment to proceed more thoroughly
We should, then, locate the onset of the general European Dark Ages from the 550-650 period. It is not entirely clear what set off this third wave of Barbarians, why the plague emerged with such consequences, why settled society proved so non-resilient, and why during the same years, Arab Bedouin proved unwilling to remain in the Arabian peninsula. Aside from fortuitous conjuncture, satisfactory explanations are elusive. One that has been offered lately to explain European, Asian, and even Meso-American changes during these century is a climatic mega-event somewhere in central Asia. An earthquake followed by a series of volcanoes spurred frantic migrations of all sorts of tribes, Avars included, into more settled areas. Also on the move was the plague. More settled areas, in turn, witnessed worsening agricultural conditions and poorer weather, perhaps set off by the seismic event. In Arabia, the event's impact on international trade was reflected in comparative impoverishment of local trade oligarchies as well as a growing gap between rich and poor both within clans and between tribes. It also made booty-oriented beduin restless, so that many would be more receptive to Muhammad's message of social justice and political expansion. Until geological historians do more research, however, this must remain conjecture.