През 1199 година Теодор се жени за [[Анна Ангелина]], дъщеря на византийския император [[Алексий III Ангел]] и [[Ефросина Дукина Каматерина]].
Theodore later distinguished himself during the sieges of [[Constantinople]] by the Latins of the [[Fourth Crusade]] (1203-1204). He remained in Constantinople until the Latins actually penetrated into the city, at which point he fled across [[Bosphorus]] together with his wife. At about the same time his brother [[Constantine Laskaris]] was unsuccessfully proclaimed emperor by some of the defenders of Constantinople. In [[Bithynia]] Theodore established himself in [[İznik|Nicaea]], which became the chief rallying-point for his countrymen.
At first Theodore did not claim the imperial title, perhaps because his father-in-law and his brother were both still living, perhaps because of the imminent Latin invasion, or perhaps because there was no [[Patriarch of Constantinople]] to crown him emperor. He was proclaimed emperor in 1205 and invited Patriarch [[John Camaterus|John Kamateros]] to Nicaea. But John died in 1206 before crowning Theodore. Theodore appointed [[Patriarch Michael IV of Constantinople|Michael IV]] as the new Patriarch and was crowned by him in March 1208.
In the meantime Theodore had been defeated by the Latins at Adramyttion ([[Edremit]]), but soon afterwards the Latins were themselves defeated by [[Kaloyan of Bulgaria]] at the [[Battle of Adrianople (1205)|Battle of Adrianople]]. This temporarily stalled the Latin advance, but it was renewed by Emperor [[Henry of Flanders]] in 1206. Theodore entered into an alliance with Kaloyan and took the offensive in 1209. The situation was complicated by the invasion of Sultan [[Kaykhusraw I]] of [[Seljuk Sultanate of Rûm|Rum]] at the instigation of the deposed Alexios III in 1211; however, the Nicaeans defeated the Seljuk army at the [[Battle of Antioch on the Meander]] where Theodore Laskaris killed the sultan in combat.<ref name="''Cambridge Medieval History'', p. 547">''Cambridge Medieval History'', p. 547</ref> Although the danger from Rum and Alexios III was thus neutralized, Emperor Henry defeated Theodore in the same year, and established his control over the southern shores of the [[Sea of Marmara]]. In spite of this defeat, Theodore was able to take advantage of the death of David Megaskomnenos, the brother of Emperor [[Alexios I of Trebizond]] in 1212 and to extend his own control over [[Paphlagonia]].