By the end of the 13th century the Asia Minor territories of the Byzantine Empire were under threat by the incursion of various Turkish tribes. In 1302 the Byzantines were defeated by the early Ottomans in the battle of Bapheus. That was the first pitched battle between Byzantium and the Ottomans. In 1303 the byzantine emperor Andronikos II Paleologos hired the Great Catalan Company, a battletested group of mercenaries, to cope with the Turks in Asia Minor. The Catalans scored a few successes against the Turks but soon they broke off with the Byzantines and turned against them. Thus, during the early years of the 14th century while the Byzantine Empire was confronting the Catalan Company in the western provinces, the Turkish occupation of Asia Minor was completed. The available sources provide several but scattered information on the military operations that took place. Byzantium faced a dire state of affairs in Asia Minor since the countryside was under the complete control of the Turks and the Byzantines were kept blockaded in fortified towns such as Nicomedia and Nicaea.
A significant degree of Turkish-Byzantine cooperation had been achieved in the city of Sardis, as the Turks had been granted a part of the city to live freely. This area was separated from the Byzantine sector with a wall. The Byzantines suspected that the Turks would change their mind and attack to fully occupy the city. As a preemptive measure they called in friendly troops. During a night the troops secretly entered the city and massacred the Turks. The byzantine emperor Andronikos II assigned the stratopedarches of the crossbowmen Sgouros and his soldiers to relieve the fortress Katoikia that was besieged by the Turks. The defenders were encouraged by the reinforcements and decided to confront the Turks in the open field. The Turks, numbering about 5,000 warriors, performed the following stratagem: they took control of the road leading to the city, where they stayed in ambush. Another part of their army attacked the Byzantines and forced them to retreat to the city walls safety via the only open escape route. But then the Turks in ambush attacked and thus the retreating Byzantines were encircled and finally defeated. As the inhabitants of Velokoma had rushed to the aid of Katoikia, the Turks took advantage of the opportunity and occupied the unguarded Velokoma.
In Tripoli of Meander, another case of “cooperation” between Byzantines and Turks has been recorded, which in this case led to the occupation of the city by the Turks. Due to the long lasting blockade and the lack of supplies, the Byzantines allowed merchants of turkish origin to enter and supply the city. After a short period, the Turks instead of supplies, loaded war trumpets on their animals and carried them into the city. During the night the merchants opened the gates and sounded the trumpets as a signal for the soldiers outside the walls to attack. The frightened and surprised Byzantines fled and the city was conquered.
The Ottomans, despite the losses inflicted on them by the Byzantine archers, captured by force the fortress of Trikkokia, near Nicaea. They filled in the moat surrounding the wall with stones, tree trunks and soil and stormed the walls. The fall of Trikkokia was followed by a massacre of the population (1307).
The ottoman chronicles outlook
The information of the Byzantine historian George Pachymeres regarding the conquest of Asia Minor and especially of Bithynia by the Turks is corroborated by some Ottoman chronicles, which preserved the related oral tradition.
(a) The Ottomans occupied Velokoma using a stratagem. They managed to put into the city some oxen loaded with big containers in which soldiers were hiding. As soon as it was night, the soldiers defeated the guards, opened the gates and the castle was occupied (it can be compared to the fall of Tripoli of Meander, as described by George Pachymeres).
(b) In order to occupy a castle in the region of Nicaea, the Turks divided their army into three parts, only one of which attacked, while the other two remained hidden, one near the fort and the other in a nearby valley. After a few days of operations, the Turks pretended to abandon the siege and the Byzantines, deceived by the false information of a prisoner, left the fortress to pursue them. At this stage the Turks near the fort occupied a gate while the rest of the turkish army counterattacked. The Byzantines retreated only to find that their way back to the city was blocked. The Turks had managed to surround them (the Turkish tactic is reminiscent of the one described by George Pachymeres that led to the fall of Katoikia).
(c) The strong fortifications of Bursa were not a match for the ottoman siegecraft and they opted for a long-term blockade combined with the construction of two “counter-fortresses” that completely cut-off the communications of the Byzantines.
The control of the countryside by the Turks, the blockade of the Byzantines in the fortified cities as well as the inability of Constantinople to send substantial reinforcements were the main causes that forced the Byzantines to surrender their cities in a process vividly described by the ottoman chronicles. The Turks were unable to occupy the fortified cities by force while the Byzantines could not count on a relief force so a fragile model of coexistence between “besiegers” and “besieged” was established. But time worked in favor of the besiegers and sooner or later the Turks took advantage of this coexistence and made use of stratagems to occupy the fortified cities. Under these circumstances it was a matter of time for whole Asia Minor to be lost for Byzantium.
 Georges Pachymérès Relations Historiques, ed. Albert Failler, Corpus Fontium Historiae Byzantinae 24, 4 vols (Paris: Les Belles Lettres, 1984 – Paris:Institut Français d’études Byzantines, 1999), IV (1999), pp. 451-59.
 Pachymérès, IV (1999), pp. 441-43.
 Pachymérès, IV (1999), pp. 455-57. George Georgiades – Arnakes, Οι Πρώτοι Οθωμανοί. Συμβολή εις το πρόβλημα της πτώσεως του Ελληνισμού της Μικράς Ασίας (1282-1337), Texte und Forschungen zur Byzantinisch-Neugriechischen Philologie 41 (Athens:1947), pp. 142-43.
 Pachymérès, IV (1999), pp. 475-79.
 Pachymérès, IV (1999), pp. 701-03.
 Elizabeth Α. Zachariadou, Ιστορία και Θρύλοι των Παλαιών Σουλτάνων (1300-1400) (Athens: National Bank of Greece Cultural Foundation, 1999), pp. 69-75.
 Zachariadou, pp.143-44.
 Zachariadou, pp. 157-58.
 Zachariadou, p. 152; Die altomanischen anonymen Chroniken, trans. by Friedrich Giese, 2 vols(Breslau: Selbstverlage, 1922 – Leipzig: F.A. Brockhaus, 1925), II (1925), pp. 19-20.
 Zachariadou, pp. 153-55, 159-60, 162-63.