Part 1

The thing about wars is that they aren't over until someone surrenders, and even then peace is not guaranteed. The Eastern Roman Empire was prepared to fight to the death in a war of attrition, the Persian nobility were not.

Reason 1 - Christianity vs Zoroastrianism

Christianity is a religion which can be utilised to prepare its followers for suffering and encourages resilience in the face of adversity. When Jerusalem was sacked in 614 and severely damaged, there was a very noticeable change in how Romans perceived the war. It stopped being another far off political squabble, and it came to be seen in religious terms. Artifacts such as the David Plates, which portrayed the struggle as a David and Goliath struggle against the Persians, and the use of relics in battle - the robes of the Virgin Mary were sometimes brought out to rally the troops - show that Christianity became a great source of strength for the remainder of the Roman Empire and for Heraclius' men. The concept of martyrdom also helped a great deal, especially in battles. Heraclius told his men that they would 'wear the crown of martyrdom' and go to heaven if they died in battle. This meant that on the battlefield, Heraclius' army fought to the end, whereas Persian resolve was not so high.

Zoroastrianism could not compete with Christianity in preparing its people for a war of attrition. I'm sure this subreddit's resident expert on Zoroastrianism can provide more detail and corrections, but Zoroastrianism was a duellist religion which cast the world as good and evil. The Persians, naturally, were good, and by extension those who opposed it were evil and should be resisted. This meant that Zoroastrians had a source of morale in their religion when on the offensive. If they really were in the right, then they ought to be winning, but that poses some pretty awkward questions if they were defeated. For many Christians, defeat meant stronger conviction in their religion - as times became more desperate clergy played an increasingly important role in fundraising, propaganda and even fighting. For Zoroastrians, defeat meant questioning the righteousness of their leader and their cause. Practically, this meant that nobles in the Shah's court would consider a coup to remove their ineffectual leader.

Reason 2 - Constantinople

As is often the case when the Byzantine Empire is severely threatened, attention must be given to Constantinople. Surrounded on three sides by water, and protected on the fourth by the mighty Theodosian walls, it was the most well fortified city in the world. The Persians had no large navy in the eastern Mediterranean, so they could not overcome the Roman fleet to move troops over to help the Avars, who were not good at sieges. Heraclius had also sent designs for artillery - including the rapid-fire ballista called a polybolos - and further fortifications to counter any attempts to get through the walls. However, such fortifications meant nothing if people were not willing to defend them. The Ecumenical Patriarch, Sergius, was key in organising defence and boosting morale through speeches, religious ceremonies, and the procession of holy relics of the Virgin Mary. This convinced the defenders that not only were they defending the heart of the empire, but fighting for God with the backing of a saint. With a highly motivated army manning some of the most formidable fortifications in the world, Constantinople’s security was fairly assured. With Constantinople guaranteed to keep the Persians and Avars busy for some time, Heraclius had the freedom to operate elsewhere.

Constantinople was also very wealthy. With the imperial treasury, the wealth of tombs and churches, and a large amount of shipping, there was actually quite a lot of money around. As the war went on, shipping dried up and tax collection became impossible as wealthy areas - Anatolia and Egypt especially - had fallen under Persian occupation. The coinage was reformed to reduce the precious metal content, and a new coin called the hexagram was introduced. The churches melted down their silver goblets and crucifixes, whilst the tombs of old emperors were looted and statues melted down to make arrowheads, armour, and ballista bolts.

Reason 3 - Persia's Political and Military Structure

Key to Roman victory was the weakness of the Sassanian army command structure. Armies were commanded not by appointed generals, but by nobles who usually had their own armies not unlike a medieval army. This meant that some nobles competed for glory and royal favour and that the Persian armies were usually more loyal to their nobleman than the Shah or the Persian state, whilst the Romans had it drilled into them that they were fighting for their civilisation. This meant that the Persians were vulnerable to being turned against one another and lured into tactically disadvantageous situations. The most effective example of this in battle came in 625 when three separate armies led by the Persian generals Shahin, Shahrbaraz and Sarablangas were lured into fighting Heraclius one by one on ground favourable to the Romans. Rather than organise themselves and share in the glory, they rushed into battle hoping to claim prestige for themselves, believing Heraclius to be disorganised. The Persian armies were seen off in succession by Heraclius' army.

As the Romans themselves were well aware, if an army was loyal to its general rather than the state, then a major rebellion could arise if a wedge were driven between general and sovereign. In 626 Shahrbaraz fell out of favour due to a failure to make significant headway in taking Constantinople, so he rebelled after an offer from the Romans. In 628, when Khusro sought a scapegoat for the defeat at the Battle of Nineveh, his generals rebelled en masse with the support of aristocrats who were tired of the war and Khusro’s inability to deal with Heraclius’ army as it ravaged the area around Ctesiphon. Khusro II had promised total victory over the Romans, yet there was a point where they could have looked out the window of his palace and seen Roman banners. This made many generals wonder why the hell they bothered, and over time loyalty to the Shah declined and developed into hostility.

Reason 4 - Heraclius' Excellent Army

In 622 Heraclius decided that he could not fight the Persians through conventional means. He decided to use a long term strategy to exploit weaknesses in Persia's political system. It is hard to say how much of what transpired was planned out from 622, but in any case Heraclius and his army exploited the Persian weaknesses very well. Heraclius recruited a new army from Caesarea and built up the confidence of the army over time through minor, and then major, incursions into Persian Anatolia. The raids of 622 were not meant to be decisive like that of 627 which provoked the overthrow of Khusro, but to build up the army’s confidence whilst securing some lost territory. They also allowed Heraclius to experiment with Maurice's Strategikon, a military treatise that Heraclius utilized to great effect throughout the 620s. When pitched battle came, Heraclius and his army had well practised strategies ready to go. Heraclius also brought Christian relics and frequently gave speeches to fortify his men with religious zeal. This meant that if the Persians attempted to foster desertion, which was a common strategy, it would not work. They were also drilled in winter and alpine combat, which gave Heraclius a lot more control over when and where he could strike. Although the Persians outnumbered the Romans, there is no doubt that Heraclius’ army was the more effective force. Heraclius also had excellent scouts, and was able to evade Persian armies as they hunted him.

Hi, I came across this thread while looking for another one, I must have forgotten about it when it was first up. Anyway, /u/J-Force has covered most of it pretty well (and I agree with his interpretation of Zoroastrianism's role in broad terms, but I think the most important aspect is the monarch's central role in it. I'm not sure the Christians were seen as "evil" as much as inferiors. Infamously, the Sasanians officially regarded the Roman Empire as a vassal state). I just have some things to add:

• As /u/J-Force alludes to the Sasanian levy was largely provided by administrative units ruled by powerful noble families that would probably have seen themselves as comptetitors with the house of Sasan.

• It is really important to note here that Heraclius never fought Khusrau's army in his counter-offensive. The surviving accounts of Khusrau's actions following Heraclius' approaching of Ganzak are difficult to make sense of; the only way I can resolve it is that his hand was forced by internal power struggles (rather than the weird account of his army disintegrating following a skirmish with Arab scouts). I am not sure this relates so much to his conduct in the war as a general dissatisfaction among the nobility following the reforms of his predecessor Khusrau I Anushirvan in the mid-6th century. Khusrau I empowered the landowning "middle class" at the expense of the ancient nobility, leading to a more effective army, but also to tensions within the realm. Following this, Heraclius sacked Ganzak, and that was undoubtedly a massive blow to the legitimacy of Khusrau (perhaps even the final straw; without it, it is hard to say what Shahin and Shahrbaraz would have done in later phases).

I don't know that we should see the tensions between Khusrau and his noble commanders as some inherent flaw in the Sasanian system as opposed to some superior system of Roman organization; I think that is a rather biased and moralizing view, and certainly Rome never had a shortage of ambitious generals creating instability (in fact, Heraclius himself was one such general, which the Sasanians took advantage of).

We should be careful about merely propagating Christian chroniclers' eulogizing Heraclius' waging of the war. I think it is also somewhat unfair to contrast the resolve of Christian armies and commanders fighting for their "civilisation" at the brink of the destruction of their empire with that of Sasanians who were and had been as concerned with internal power struggles as external ones. The beginning of the campaign and the battle of Antioch shows how effective the Persians could be, both in terms of strategy and tactics. In closing I don't think the later phase of the war damns the Persians any more than the earlier phase damns the Romans.

Touraj Daryaee's Sasanian Persia is a good overview of Sasanian history, if you want a broader perspective.