In 1254 the great emperor John III Vatatzis died, leaving the Empire of Nicaea powerful. John had succeeded not only in fending off the attacks of the Latins against him, but also in liberating many lands in Thrace and Macedonia. At the same time, he had faced the Turkish and Bulgarian danger.
His son and successor, however, was about to find himself having to face again the expansionism of the Bulgarians. Ioannis Vatatzis had defeated the Bulgarians and had imposed his terms on them, sealing the agreement with the marriage of his son Theodoros with his daughter Bulgarian tsar Ioannis Asan, Eleni. After the death of Ioannis Asan, the Bulgarian throne was occupied by his son Michael Asan, brother of Empress Helen.
He did not disturb the Empire until the moment John Vatatzis died. Then he considered the moment suitable to recover all the lands that the Bulgarian kingdom had lost from John Vatatzis. The Bulgarian king estimated that his successor, Theodore II, would need some time to establish himself on his throne.
He also took advantage of the fact that at that time there were no serious imperial forces in Macedonia and Thrace, with the exception of the few garrisons in the various cities. Finally, he counted on the factor of surprise, since the relations between the two states, at that time, were satisfactory and there were kinship ties between the two royal houses, so the Byzantines did not expect a Bulgarian attack.
The Bulgarian king, however, did not attack immediately. He waited until winter to attack, so that it would not be easy to supply the various garrisons with food – in winter the army could not be fed from the ententes – but also that an immediate Byzantine reaction would not be possible. At least that's what he thought.
Bulgarian invasion
Michael Asan gathered his forces north of the river Ebro, which formed the border between the two states, in December 1254 and invaded the imperial territories in early January 1255. The Bulgarians moved, literally, at lightning speed, despite the bad weather conditions and quickly overwhelmed many cities, villages and settlements, almost without resistance.
Most of the Imperial garrisons surrendered the cities they held with the agreement to be let go freely, knowing that without food they could not hold out for long. After all, several cities and villages were also inhabited by Bulgarians and the Byzantine garrisons could not fight both an external and an internal enemy.
Into the hands of Michael Asan then fell Stenimachos - today's Asanovgrad, a Greek city of what was once Eastern Romilia - the fortresses of Peristitza, Krytzimos (today's Chrysim, 20 km southwest of Philippopolis) and Tzepaina (in the northwest Rhodopes) and the whole area up to Lake Ahrida, with the exception of the small town of Mniakos, the only one that remained under Byzantine control. The Bulgarians captured other smaller towns and fortresses such as Ustra, Perperakion and Kryvo, reaching as far as the outskirts of Andrianoupolis.
After these successes, Michael Asan began the effort to consolidate Bulgarian sovereignty in the territories he occupied. He placed strong garrisons in the cities and fortresses and supplied them with weapons and food - which he had collected - so that he would be able to face the Byzantine counter-attack - which he calculated for the spring of 1255. But he calculated without his courage and tenacity Theodorou.
The Battle of the Key
An even more serious threat to Theodoros was the Bulgarian counter-distraction operation, in the area of Tzoumaia, against the small fortified town of Meleniko. Significant Bulgarian forces marched against the city, under General Dragan (Dragotas according to G. Akropolitis – probably of Serbian origin).
The small Byzantine garrison was headed by Ioannis Nestogos and Ioannis Angelos. The Bulgarians repeatedly attacked the small town, but were repulsed with heavy losses. Emperor Theodore, when he was informed of the Bulgarian attack on Meleniko, decided to act immediately. He assembled his army and, leaving the transports behind, marched, in the heart of winter, in 12 days, to Serres.
The horsemen had loaded their horses with provisions and food for the whole army. After resting his army for a day at Serres, the emperor marched towards the strait of Rupel (Byzantine Rupelius). The Bulgarians, of course, had blocked the narrow passage with a wall and had stationed there a numerous foot guard, which was also supported by a few horsemen.
The emperor, when he reached the strait, which in its narrowest part was about three meters wide – also due to the bed of the river Strymons – understood that a frontal attack would only cause unnecessary damage to his forces and nothing else. That is why he decided to follow the example of Basil II Bulgaroktonos and his general, Nikephoros Uranus, during the famous battle of Cleidius, in 1014. The difference was that the battle of Cleidius was fought in July, whereas now it was February and the snow was close to a meter thick!
Theodoros was not one to be easily disappointed, however. He immediately realized that he had to outflank the Bulgarian position and the most suitable men for this mission were his minions, his lightly armed infantry, that is.
Thus, while he himself with the cavalry and heavy infantry marched in front of the Bulgarian fortifications and began to harass the Bulgarians with archery and small attacks, the light infantry, as scouts of the time , climbed the forested slopes of Mount Agistrou, where the Rupel fort would later be built.
When the light infantry reached the appropriate positions the signal was given and the Byzantine Army charged against the Bulgarians. The latter, seeing before them, the Byzantines, prepared to face them, rather pleased by the "foolishness" of their opponents to attack them head on. But suddenly arrows and javelins began to hit them on their left side. Terrified then, they found they had been outgunned and fled en masse. The horsemen managed to escape. But their infantry was neutralized, as a whole.
The Bulgarian horsemen who escaped joined a body of the Bulgarian Army, commanded by Dragotas, who was sent there to stop the Byzantine advance and which was stationed further north, spreading panic there as well.
The Bulgarians fled and in the moonless night many were killed by falling into ravines or trampled by their panic-stricken comrades. Thus also died Dragotas, who fell from his horse and was trampled by the horses of his fellow warriors. He died, in excruciating pain, three days later.
After his victory, Theodore moved towards Meleniko forcing the Bulgarian besiegers to flee in turn. After this the emperor first returned to Thessaloniki and then moved towards Vodena (today's Edessa).
There, however, Theodoros fell ill and remained, obligatorily, for a few days. When he recovered he moved to the city of Prilapos, which he made a base of operations in western Macedonia. When his siege engines also arrived, the emperor marched with his army towards Velesos (present-day Veles in the state of Skopje, formerly Tito Veles).
Theodore arrived before the city and prepared to besiege it. However, the Bulgarian garrison, at the sight of his siege machines, became paralyzed and surrendered with a treaty. About 500 Bulgarians were allowed to go free. Then the emperor turned east and arrived at Stromnitsa (the ancient Greek city of Astraion) of today's Skopje and from there he marched to Melenikos and returned to Serres.
In this way, Theodore liberated almost all the territories occupied by the Bulgarians, with the exception of Tzepaina and the small fortress of Patmos, in the Rhodopes – the latter was eventually captured by the general Alexios Philanthropinos.