In 894 Stylianos Zaoutzes, basileopator and leading minister of Leo VI the Wise (r. 886–912), convinced the emperor to move the Bulgarian market from Constantinople to Thessaloniki.
That move affected not only private interests but also the international commercial importance of Bulgaria and the principle of Byzantine–Bulgarian trade, regulated with the Treaty of 716 and later agreements on the most favoured nation basis.
The Bulgarian merchants were allowed to live in Constantinople, resided in their own colony and paid favourable taxes.
The city was a major destination of trade routes from all over Europe and Asia and the transfer of the Bulgarian market to Thessaloniki cut short the direct access to goods from the east, which under the new circumstances the Bulgarians would have to buy through middlemen, who were close associates of Stylianos Zaoutzes.
In Thessaloniki the Bulgarians were also forced to pay higher tariffs to sell their goods, enriching Zaoutzes' cronies.

The Byzantine chronicler Theophanes Continuatus described the reasons for the conflict as follows:
The cause of the war was the following — the basileopator Stylianos Zaoutzes had a eunuch slave named Mousikos.
He befriended Staurakios and Cosmas, who originated from Hellas, merchants greedy for profit and covetous.
In their desire to enrich themselves and through the mediation of Mousikos, they moved the market of the Bulgarians from the capital [Constantinople] to Thessaloniki and taxed the Bulgarians with heavier duties.
When the Bulgarians acquainted Simeon with the issue, he informed emperor Leo.
Infatuated in his predilection to Zaoutzes, he considered all this a trifle.
Simeon was infuriated and raised arms against the Romans.

—Chronographia by Theophanes Continuatus
The ousting of the merchants from Constantinople was a heavy blow for Bulgarian economic interests.
The merchants complained to Simeon I, who in turn raised the issue to Leo VI, but the appeal was left unanswered.
Simeon, who according to the Byzantine chroniclers was seeking a pretext to declare war and to implement his plans to seize the Byzantine throne, attacked, provoking what has sometimes been called (inappropriately) the first commercial war in Europe.
However, many historians including Vasil Zlatarski and John Fine consider those claims unlikely, arguing that in the beginning of his reign Simeon needed to consolidate his power and imperial ambitions had not yet been crystallised, therefore his military intervention was a defensive act to protect the Bulgarian commercial interests.
In 894 Stylianos Zaoutzes, basileopator and leading minister of Leo VI the Wise (r. 886–912), convinced the emperor to move the Bulgarian market from Constantinople to Thessaloniki. That move affected not only private interests but also the international commercial importance of Bulgaria and the principle of Byzantine–Bulgarian trade, regulated with the Treaty of 716 and later agreements on the most favoured nation basis. The Bulgarian merchants were allowed to live in Constantinople, resided in their own colony and paid favourable taxes. The city was a major destination of trade routes from all over Europe and Asia and the transfer of the Bulgarian market to Thessaloniki cut short the direct access to goods from the east, which under the new circumstances the Bulgarians would have to buy through middlemen, who were close associates of Stylianos Zaoutzes. In Thessaloniki the Bulgarians were also forced to pay higher tariffs to sell their goods, enriching Zaoutzes' cronies.
The Byzantine chronicler Theophanes Continuatus described the reasons for the conflict as follows:
The cause of the war was the following — the basileopator Stylianos Zaoutzes had a eunuch slave named Mousikos. He befriended Staurakios and Cosmas, who originated from Hellas, merchants greedy for profit and covetous. In their desire to enrich themselves and through the mediation of Mousikos, they moved the market of the Bulgarians from the capital [Constantinople] to Thessaloniki and taxed the Bulgarians with heavier duties. When the Bulgarians acquainted Simeon with the issue, he informed emperor Leo. Infatuated in his predilection to Zaoutzes, he considered all this a trifle. Simeon was infuriated and raised arms against the Romans.
—Chronographia by Theophanes Continuatus
The ousting of the merchants from Constantinople was a heavy blow for Bulgarian economic interests. The merchants complained to Simeon I, who in turn raised the issue to Leo VI, but the appeal was left unanswered. Simeon, who according to the Byzantine chroniclers was seeking a pretext to declare war and to implement his plans to seize the Byzantine throne, attacked, provoking what has sometimes been called (inappropriately) the first commercial war in Europe. However, many historians including Vasil Zlatarski and John Fine consider those claims unlikely, arguing that in the beginning of his reign Simeon needed to consolidate his power and imperial ambitions had not yet been crystallised, therefore his military intervention was a defensive act to protect the Bulgarian commercial interests.
Byzantine–Bulgarian war of 894–896