After marching from Georgia, and with the help of their paternal relative Queen Tamar, Alexios and David occupied Trebizond in April 1204. That same month Alexios was proclaimed emperor at the age of 22, an act considered by later writers as the moment the Empire of Trebizond was founded. Vasiliev was one of the first historians to suspect that Tamar assisted her young relatives for reasons beyond familial loyalty. "Religiously minded, Tamar had the habit of bestowing alms on monasteries and churches not only in her own country but also all over the Near East", Vasiliev wrote in 1936. One such gift she bestowed on a group of monks before they left for Jerusalem was taken from them by the Byzantine emperor
Alexios III Angelos (r. 1195–1203) as they sailed past Constantinople; although Tamar subsequently made up for the theft by giving the monks a much more lavish gift, Alexios' theft insulted the queen. She decided to avenge the insult by supporting her nephews in their invasion of Byzantine territories. The date Alexios entered Trebizond may be narrowed down even further.
Sergey Karpov has identified a lead seal of Alexios, on one side "the image of a
strategos in the peaked helmet leaded by hand by St. George" with the inscriptions Ἀλέξιος ὁ Κομνηνός [Alexios Komnenos] and Ὁ Ἅ(γιος) Γεώργιος [Saint George] on either side; on the obverse is a scene of Ἡ Ἁγία Ἀνάστασις [The Holy Resurrection] with the corresponding inscription. Karpov interprets the significance of this image and the inscription as portraying the most important achievement of his life, St. George inviting the victorious prince to enter Trebizond and opening the gates of the city with his left hand. The importance of St. George was that Easter—the date of the Resurrection—in 1204 fell on 25 April, while the memorial date of St. George was 23 April. "So I dared to assume," writes Karpov, "that the seal points out the date of the capture of Trebizond." Vasiliev points out that the brothers occupied Trebizond too early to have done so in response to the Crusaders capturing Constantinople; Alexios and David began their march on Trebizond before news of the
sack of Constantinople on 13 April 1204 could reach either Trebizond or Georgia. According to Vasiliev, however, their original intention was not to seize a base from which they could recover the capital of the Byzantine Empire, but rather to carve out of the Byzantine Empire a
buffer state to protect Georgia from the
Seljuk Turks. Kuršanskis, while agreeing with Vasiliev that Tamar was motivated by revenge for Alexios Angelos's insult, proposed a more obvious motivation for the brothers' return to Byzantine territory: they had decided to raise the banner of revolt, depose Alexios Angelos, and return the imperial throne to the
Komnenos dynasty. However, not long after they had gained control of Trebizond and the neighboring territories, news of the Latin conquest of Constantinople reached them, and the brothers entered the competition for recovery of the imperial city against
Theodore I Laskaris in western Anatolia (ruler of the "
Empire of Nicaea") and
Michael Komnenos Doukas in mainland Greece (ruler of the "
Despotate of Epirus"). Over the following months, David marched westward making himself master of the rest of the Pontus and of
Paphlagonia. Anthony Bryer suggests the account in the Georgian Royal Annals of the invasion could be separated into the two routes the individual brothers took. Both started in
Imereti and reached Trebizond; David proceeded along the coast, perhaps leading a fleet, capturing
Kerasous,
Cide,
Amasra and
Heraclea Pontica; meanwhile Alexios took possession of
Limnia,
Samsun and
Sinope. Although a minor port, Samsun was the Seljuk
Sultanate of Rum's doorway to the
Black Sea, and Alexios' occupation blocked the Sultanate from the trade and the opportunities of expansion Samsun represented; in the words of the Muslim historian
Ali ibn al-Athir, he "closed the sea" to the Seljuks. Gaining Paphlagonia gave the brothers access to an important base of support. The Komnenos family was popular in Paphlagonia, with which they had long-established ties, as it was their home province:
Kastamone was said to be the ancestral castle of the Komnenoi; during the reign of Isaac II Angelos a pretender to the throne had appeared in Paphlagonia, calling himself
Alexios, and he succeeded in uniting several districts behind him. While David was in Paphlagonia, Alexios was forced to remain in the neighborhood of Trebizond, defending the eastern part of their domain from the attacks of the Seljuk Turks. These attacks culminated in the
first siege of Trebizond by Sultan
Kaykhusraw I. In a panegyric to his master, the Nicaean emperor Theodore Laskaris,
Nicetas Choniates compared Alexios to
Hylas, a member of the expedition of the
Argonauts who landed on the coast of
Mysia to obtain water, but was kidnapped by the
Naiads and never seen again. Although Theodore Laskaris pushed back the Komnenos brothers' western frontier by defeating an attempt to seize
Nicomedia, by 1207 the grandsons of Andronikos Komnenos ruled over the largest of the three Byzantine successor states. From Heraclea Pontica their domain extended east to Trebizond and past it to
Soterioupolis on the Georgian frontier. Alexios also made parts of the
Crimea a tributary to Trebizond.
Cherson,
Kerch, and their hinterlands were governed as an overseas province called
Perateia ("beyond the sea"). It appeared that it was only a matter of time before one of the Komnenos brothers seized Constantinople to rule as "
Basileus and
Autokrator of the
Rhomaioi". Unfortunately, this proved to be the high-water mark of their conquests. == Campaigns in Paphlagonia ==