In 1256, he repelled a Bulgarian invasion of Thrace and Macedonia and forced Michael II Komnenos Doukas, the ruler of Epirus, to cede Dyrrachium on the coast of the Adriatic Sea to Nicaea.

Theodore was born to Emperor John III Doukas Vatatzes (r. 1221–1254) and Irene Laskarina in the imperial palace in Nicaea in late 1221 or early 1222.

The battle weakened Epirus–Thessalonica, Nicaea's western rival for the revival of the Byzantine Empire, and made Bulgaria the dominant power of the Balkan Peninsula.

In 1237, Ivan Asen then reversed his position, allying himself with the Latins to stop Nicaean expansion in Thrace, only to return to his alliance with Nicaea before the end of the year.

As Blemmydes was the hegumenos (abbot) of a monastery at Ephesus, Theodore attended his classes while he was staying in his father's winter palace at nearby Nymphaion.

[14] Vatatzes remarried in late summer of 1240, taking an illegitimate daughter of Frederick II, Holy Roman Emperor (r. 1220–1250), Constanza, whose name was changed to Anna.

[18] At this time, a Mongol general, Baiju, invaded Nicaea's eastern neighbor, the Seljuk Sultanate of Rum, from the east and captured Erzurum.

Blemmydes did not tolerate sexual misconduct, and completed a mirror for princes (instruction), entitled Imperial Statue, warning both Theodore and his father against lechery.

[24] Vatatzes was making preparations for a new attack on Constantinople in the spring of 1252, but the ruler of Epirus, Michael II Komnenos Doukas (r. 1230–1266/68), invaded Nicaean territory in Macedonia.

[25][26] Early in 1253, Emperor Frederick II's son and successor, Conrad (r. 1250–1254), forced most relatives of his father's mistress, Bianca Lancia, into exile.

In his funerary speech at Vatatzes' burial, Akropolites expressed his hope that Theodore would lead the Nicaeans to expel the Latins from Constantinople, comparing him with the "pillar of fire" of the Book of Exodus, guiding the Israelites.

[36] Theodore did not trust the aristocrats and wrote of "illicit love affairs and very unjust plots", without naming his opponents, in a letter addressed to George Mouzalon in December 1254.

The historian Michael Angold proposes that Emperor Frederick II's similar policy may have inspired Theodore's "more autocratic stance towards his aristocracy".

He recalled his mother's uncles, Michael and Manuel Laskaris, from their exile and made two members of the rebellious Nestongos family, George and Isaac, generals.

His surprise attack forced the invaders to withdraw from Thrace, and he ordered two Nicaean aristocrats, Alexios Strategopoulos and Demetrios Tornikes, to pursue the Bulgarians.

Theodore closed down the local mint, because he wanted to establish a centralized system of financial administration in Asia Minor, with a new treasury at Astritzion on the Asian coast of the Hellespont.

He invaded Macedonia and forced the Bulgarian garrison in Veles to surrender, but drought prevented him from continuing the military campaign across the arid Ovče Pole.

He made Manuel Laskaris and Constantine Margarites the commanders-in-chief of his troops in Thrace and charged George Mouzalon with implementing military reforms.

Although Theodore had instructed Laskaris and Margarites to adopt a defensive strategy, they pursued the Cumans, but the invaders inflicted a heavy defeat on their troops.

[39][48][49] The peace treaty aroused much indignation in Bulgaria, enabling Michael II's cousin, Kaliman (r. 1256), to stage a plot and dethrone the Tzar.

The medical historian John Lascaratos and the neurologist Panaghiotis Zis maintain that Pachymeres' diagnosis was correct, concluding that Theodore developed epilepsy of the tonic-clonic type in his late twenties.

[61] The historian Donald M. Nicol proposes that Theodore's illness "made him the prey of his moods, and he compensated for his nervousness by an autocratic and obstinate temper".

The commanders of the army in Macedonia, most of them of humble origin, recently appointed by Theodore, could not prevent the Epirotes, Albanians and Serbians from raiding Nicaean territories.

Michael II was planning to launch an attack against Thessalonica, but Manfred of Sicily renewed his predecessors' claim to Byzantine territories in the Balkans and invaded Epirus from the west.

Michael II, who did not want to abandon his plan, concluded an alliance with Manfred who expelled the Nicaean garrisons from Dyrrachium and other fortresses on the Adriatic by the end of February 1258.

Palaiologos convinced the aristocrats that an empire ruled by a boy could not resist enemy attacks, and he was proclaimed co-emperor as Michael VIII (r. 1259–1282) before the end of the year.

[71] To emphasize that the Laskaris family had lost their imperial status, Michael VIII married off John's three sisters to minor Italian and Bulgarian noblemen.

[72] A man of letters devoted to philosophical and theological studies, Theodore transformed his court into a center for renowned scholars shortly after his accession to the throne.

[36][73] During his father's lifetime he began to write treatises on philosophical, theological and historical subjects, but decided to prepare manuscript editions of his works only after his meeting with Berthold of Hohenburg in 1253.

[76] Neither Theodore nor Elena had reached the minimum legal age of marriage—fourteen for boys and twelve for girls—when they were married, but such an early marriage was not unique among Nicaean aristocrats and commoners.