After crossing the Bosporus into Asia Minor (in present-day Turkey), he started to organise the local Greeks' resistance against the Latins in Bithynia in his father-in-law's name.

Neither could he prevent a claimant to the imperial throne, Alexios Komnenos, from establishing a Byzantine successor state, the Empire of Trebizond, in northern Asia Minor.

Theodore's position consolidated only after Tzar Kaloyan of Bulgaria inflicted a crushing defeat on the Latins in the Battle of Adrianople (in Thrace) in 1205.

Greeks fleeing from the Latin Empire—the crusader state that emerged in the Byzantine core territories—swarmed to Asia Minor to live under Theodore's rule.

The Orthodox population of the Latin Empire regarded Theodore as the main defender of their Church, but the rulers of Epirus—a realm that developed in the western regions of the Byzantine Empire—debated the legality of his coronation.

The Latin Emperor Henry of Flanders captured important fortresses in western Asia Minor in 1212, but he ran out of soldiers and could not place garrisons in them.

Theodore established a powerful state, located in the vicinity of Constantinople, which enabled his successors to expel the Latins from the city and revive the Byzantine Empire in 1261.

[9] The slightly later historian George Akropolites recorded that Theodore was "small in body but not excessively so, quite dark, and had a flowing beard forked at the end".

[9] To seize the Byzantine throne, Emperor Alexios III had blinded and imprisoned his elder brother, Isaac II Angelos (r. 1185–1195).

[16][17] The details of his escape are unknown, but Choniates stated that Theodore left Constantinople "armed only with practical wisdom and a brave spirit".

[16][22] Isaac II had already died, and the new emperor had Alexios IV murdered, providing the crusaders with an excuse to make a new assault on Constantinople again.

[28] A rebellious magnate, Theodore Mangaphas, held Philadelphia; another aristocrat, Sabas Asidenos, ruled Sampson; and Nikephoros Kontostephanos controlled the lands on the upper course of the Maeander River.

[31] He established his seat in Prussa,[32] but he made frequent journeys to attend assemblies and dinners, encouraging the local Greeks' resistance against the "Latins", as the crusaders were universally called.

He also took control of state revenues, and he could offer money to the Seljuq Sultan of Rum, Rukn al-Din Suleiman II, in return for his assistance against the Latins.

[21] The Venetians seized the port of Lampsacus on the Asian side of the Hellespont and a French knight, Peter of Bracieux, captured the nearby Pegai.

[21] Emperor Alexios I of Trebizond's brother and co-emperor, David Komnenos, launched a military campaign into Paphlagonia and occupied the towns along the Black Sea coast.

[42] Theodore approached Patriarch John Kamateros, who lived in exile in Thrace, offering to move him to Nicaea, but the elderly prelate refused.

[48] His wife's uncle, the blind Basil Doukas Kamateros—a former logothetes tou dromou (minister of foreign affairs)—assisted him in setting up the new administrative system.

Theodore in turn allied himself with Kaloyan, who launched an incursion into Thrace, forcing Emperor Henry to recall his troops from Asia Minor.

Theodore's opponents argued that the council electing Michael Autoreianos was just an assembly of randomly chosen bishops, not a properly convoked synod.

[52] Alexios I and David Komnenos consolidated their Empire of Trebizond in northern Asia Minor; Henry of Flanders integrated Thrace and almost all of Greece into the Latin Empire; Michael I Doukas secured his rule in Epirus; and Theodore I Laskaris emerged as the unrivaled ruler of western Asia Minor.

[61] Both Boril of Bulgaria and Michael I Doukas wanted to expel the Latins from Thessalonica, prompting Emperor Henry to visit the town regularly.

[65] Kaykhusraw I and Alexios III invaded Nicaea, forcing Theodore's troops to abandon the siege and hurry back to Asia Minor.

In a circular letter sent to the European monarchs early in 1212, he boasted of having subjugated the Greeks as far as the Seljuq frontier, save the garrisons of some fortresses.

[67] The Latins also seized some strategically important Bithynian fortresses,[note 6] taking control of the roads between the northern and southern territories of Theodore's realm.

[30] His conquest of the region put a narrow strip of land along the Black Sea coast under Nicaean control,[71] and thus removed the emperors of Trebizond from the competition for Constantinople.

[66] The Latin clerics wanted to compel the Orthodox population of Constantinople to adopt the Catholic liturgy and to pay the tithe, but they resisted.

[45] In concert with Demetrios Chomatenos, the ambitious Archbishop of Ochrid, Doukas denied the right of the patriarch residing in Nicaea to appoint bishops to the Balkan episcopal sees under Epirote control.

[90] Theodore realized that he was unable to recapture Constantinople from the Latins,[87] but located near the old Byzantine capital, Nicaea was ideally placed for its future reconquest.

[37] Historian Dimiter Angelov emphasizes that Theodore's political success was "due, in no small part, to his sanguine and pragmatic approach".