Before the Rus raid, Theodosius continued to move west but suffered a defeat in an ambush in Pelagonia, where his rearguard was destroyed. Still, he liberated most of Greece. His father, after the Slavs were defeated in Thessalonica, now moved and laid siege to Serdica, which he defended. By this point, the soldiers were frustrated, and after repelling a massive assault on the walls, David was declared emperor by his troops. He again denied this, but his troops were about to force him to march towards the capital; hence, he told them to wait until the emperor left and then they would march towards Constantinople.
This trick worked and delayed them enough, and soon news came that the emperor was coming. Constantine left the capital with 50,000 men, along with Theodosius' and David's armies and others. Now that 1/3 of the imperial army was in the Balkans, Simeon, who was near Philippopolis, quickly left for Bulgaria, for he knew the romans would not chase him north as they had taken nearly all the major forts in the river. The rebels had a dilemma as well. On the one hand, one wanted to ambush the emperor near the Trajan gates, but others wanted to retreat. In the end, timidity won the day. As they left, news came that the emperor's forces were near by. Some were scared, while others hoped David would lead them to victory. But there would be no battle or siege. When he arrived, the emperor was not happy with his cousin's proclamation, but David assured him that he would never be like his father in that regard and explained the situation to him, which Constantine understood.
Thus the emperor divided his force: 20,000 would stay and move if Simeon came to raid; they were told not to engage unless they had a clear advantage and just chase the emperor north of the Danube. The rest of the men, including David, followed the emperor, but instead of listing to his commanders and moving towards Croatia, the emperor marched towards Scupi. David realized what he was doing: he would liberate Macedonia before going for the heart of the rebellion. The reason made a lot of sense: the rebels had massacred their way south. They had not only challenged imperial power but had also caused the population to doubt the emperor by saying he didn't care or was a coward, which he could not allow.
Thus Constantine's massive army moved towards Scupi; scouts were constantly sent to avoid any ambush. Once near the city, Vladimir was on the other side of the river with his main force and an ambush, but it was not to be. David found a shallow part of the river, crossed it during the night, and attacked the camp. It is said that 12,000 rebels died with the victory, and the emperor entered the city just as his name was cheered in Thessalonica; it too was cheered in Scupi, then he moved south.
Surprisingly, the emperor showed mercy to the population that had supplied the Romans; he knew most of them did not do it willingly, and even those who did were pardoned. His mercy was not limited to the Greeks; the Slavs who had joined them were also pardoned and given some gifts. This did not compare, however, to the many Slavs who remained loyal, as they were given gifts and promised more once the rebellion ended. Constantine also met Andrei the Belegezite, to whom he said, Is it true what they say about you, oh brave ruler, that it was this bravery that saved Thessalonica?
To which Andrei replied, Oh, my emperor, son of the Heraclii, I assure you, one man's death was inconsequential to the fate of that great city.
As Theodosius moved east, he was ambushed and still suffered losses, but by the end of the year, the rebels had retreated and were dislocated from their fortified positions in the mountains of Macedonia. Stephen was now worried that he had used the time to fortify his position. In Dacia and Dalmatia, however much the emperor's presence offered a solution, his death could lead to civil war.
By 911, the emperor had left Scupi, and he again divided his massive force of 15,000 to go north to the Danube as the emperor moved along the coast before he went inland towards Doclea, which he took once he crossed the territory of the Docleans. His attitude changed from mercy to retribution as he started to kill and enslave every single Slav he found, and he said, Give to them double.
Any Slavic leader captured was flogged until he confessed that the rebel armies were hiding in the mountains. Many were still put to death, and many were enslaved, Constantine continued to cut a bloody path, and soon he was joined by David, who also committed similar massacres of Danubian Slavs despite capturing Ratiaria. Simeon used the rebels' weaknesses to capture them from the fort of Sirmium to the fort of Aquis. Not having the numbers to deal with Simeon David, who returned to his emperor, they kept to the coast as the attempted raids into the hinterlands noticed that they were empty. The scorched-earth tactics meant that the emperor's army would die of starvation, so he continued to march, and soon his vassal moved troops from northern Italy to the Balkans, taking the same route the legendary eunuch Narces did 360 years ago.
As the Roman forces from the south looted and destroyed any village on their way, in June 911, they reached Salona, where both Vladimir and Stephen were desperate, especially the king of the Slavs, as victory here would save his kingdom. Not wanting a siege, he sallied out to face the Roman forces, which, according to all sources, had 60,000 men, some professional soldiers, others conscripts; the Romans had about 50,000.
Before the battle, the king gave a speech.
“Men, I am a descendant of the great kings, but today I do not fight for my wealth nor my title but rather for not only my freedom and life but yours as well. Consider that even if you flee, you shall live in slavery, but I shall not be any slave, so men, I beseech you to join me in victory or death”
The emperor also addressed his troops: Men, we have already overcome our most formidable odds; have we not encountered hunger and the savagery of these barbarians? But do know that today everything will be decided. Remember what you promised when we parted ways with Serdica, that you would not return beyond the Drava unless it was as victors? Let us spare not a single one for the slaughter of our allies and our kindred to strike fear in you, but you have chosen victory or death, so go forth and conquer.
The Slavs charged with their infantry in the center, cavalry and archers in the wings, as the Romans also had their infantry in the center and calvary in the wings, and so the battle began. The fighting was fierce, and the ferocity of the Slavs matched the discipline of the Romans. As men died by spear or sword, the Slavs continued to push forward, while the archers in the wing prevented the Roman cavalry from outflanking them. This continued for some time as the Slavs slowly but surely pushed the Romans back. The contest was bloody, and now the king went with his reserve and pushed the Romans back, but not enough to break them. Now it was the Romans turn. The emperor unsheathed the sword of Heraclius, and with his cavalry guard, he charged The charge and the troops morale were enough to push them back to the city. Slowly but surely, the Slavs were carved, and just before reaching the city, the vigor of the Slavs broke. Many fled towards the city, others to the hills.
Among them was the king of the Slavs, convinced by Vladimir to flee. Gregory claims that the Romans had to pass over 25,000 corpses to get to the city. Those who had fled to the city, led by Vladimir, resisted for two hours as the siege soon ended. Salona, which was purged of its Roman population, was now purged of its Slavs. Even the animals were killed, by the end of the day; 60,000 had died, including the entire population of Salona, but Constantine was not done; he now marched his victorious army to the capital of the rebel kingdom, and Stephen prepared his capital for a siege. By now, many nobles had fled the city as the king became paranoid, and others knew what awaited them when the Romans arrived. Those who were left killed their king, hoping to receive mercy, but once Constantine came, their hopes died.
The locals tried to desperately fight with children throwing stones, but this last defiance was snuffed out quickly. What followed next was a bloodbath, as Gregory wrote: The soldiers spent time killing and searching; the men were tired, and yet there appeared to be a vast multitude still remaining alive. Their compatriots then entered the city, taking turns to kill anyone they found; they slew the aged and the sick, but for those that were in their flourishing age, they enslaved them along with many young women; others drove them together and shut them up within the walls of the palace, which was burned; the blood ran up to the scabbard of the sword of Heraclius and the Basileus knees on top of his horse.
The emperor also captured ten members of the Croatian ruling dynasty, and the five senior ones were tortured to death by anyone related to them, or even the long line of Serb princes was banned from setting foot in the city and taking a political position in Illyria. The Roman forces now moved south to the hinterland he avoided; now towns were erased from existence, and by September 911, the revolt was crushed but the actions of the soldiers did not stop Constantine returned to Constantinople as a hero, but not by all, as some, mainly the clergy, were horrified by the emperor's actions. Thousands of Christians, mostly non-combatants, had been killed or sold into slavery.
But Constantine did not care, despite the fact that, according to Gregory and Sophia's secret history, 10 major cities and 500 towns had been destroyed and nearly 700,000 Slavs had died or been enslaved; nearly half of the deaths would be from famine and disease. So many had died that the pagan Slavs had disappeared, not by religious persecution, as the Roman soldiers did not discriminate, but by in the emperor eyes was retribution for the 200,000 Romans, Greek Latin or loyal Slavs who had died during the revolt, Constantine, as he promised, would reward the loyal Slavs by giving them titles and fixed salaries. Andrie, for example, was given the title of Patrikos and was even invited to the Byzantine senate, but for his enemies, the emperor was not done; he passed new laws targeting the Slavs of the north, Greek or Latin was to be used in church service; church Slavonic was banned; writing the pagan Slavic myths or even their history was outlawed; and those texts that existed were either confiscated or burned, the tribes were to be split, and many would be moved to the east; the rest were sent to the slave markets.