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Historically, Thessaloniki is one of the most important cities in Europe, though it's probably never been quite as prominent as its importance should have warranted because it's generally would up being overshadowed somehow. Founded in the fourth century BC by Macedonian King Cassander, it rose in stature to become the most important city in Macedon...shortly before the Romans showed up. It became an important Roman city...because it was a way point on the main route from Rome to Byzantium. When Diocletian divided the Roman Empire into the Tetrarchy in the third century, it became one of the capital cities of the empire under eastern Emperor Galerius...and then Constantine ended the Tetrarchy and established a new eastern capital, Constantinople. It was the "second city" of the Byzantine Empire. Today it's the second largest city in Greece. You get the idea.
Despite this perpetual "second city" status, Thessaloniki's importance was not lost on the Ottoman Empire in its quest to establish itself across the Bosphorus in Europe. In fact, the Ottomans took Thessaloniki in 1387, and it quickly became their most vital European possession...and then the empire lost almost everything at the Battle of Ankara in 1402. Overshadowed again. In the Ottoman civil war that followed Ankara, the Byzantines were able regain Thessaloniki in return for supporting the imperial claims of Süleyman Çelebi (d. 1411). But this temporary chaos masked the fact that, even after Timur had taken their empire completely apart, the Ottomans were still stronger than whatever was left of the Byzantine Empire by the early 15th century.
When Mehmed I (d. 1421), rather than Süleyman Çelebi, emerged from the interregnum in 1413 as the ruler of a once-more unified Ottoman principality, the Byzantines very quickly found themselves back behind the proverbial eight ball. They'd backed the wrong horse, as it turned out, and their claim on Thessaloniki wasn't something Mehmed felt compelled to honor. The Ottomans almost retook the city in 1416. After Mehmed had defeated three of his brothers and established his rule in 1413, yet another brother, Mustafa Çelebi (d. 1422), returned from imprisonment in Central Asia (courtesy of Timur) thinking he and Mehmed should at least split the empire between them. Mehmed disagreed, and the two men and their armies met in battle. Mustafa lost fled to Thessaloniki. Mehmed could have besieged the city at that point, but instead he cut a deal with Byzantine Emperor Manuel II Palaiologos (d. 1425) that let the Byzantines keep the city in return for also keeping Mustafa in exile, on the island of Lemnos.
Murad II (Wikimedia Commons)
Taking Mustafa in bought the Byzantines a little time, and it also--as far as they were concerned--bought them control over somebody who could genuinely become the new Ottoman ruler when Mehmed died. Which he did, in 1421, with the intention that he be succeeded by his son, Murad II (d. 1451). The Byzantines, of course, wanted their captive on the throne, and so they released Mustafa from his exile and brought him to the mainland, where he was able to recruit an army and capture Adrianople (modern Edirne), then the Ottomans' European capital. At this point, though, Mustafa got a little big for his britches, tried to invade Anatolia, and, well, long story short, Murad had him executed in the spring of 1422. The Byzantines had, remarkably, managed to back the wrong horse a second time.
You can see Thessaloniki (here "Saloniki") in the northern Greece area
At this point Murad vowed to crush the Byzantine Empire in revenge for the aid they'd given Mustafa. That vow would actually be fulfilled by his son, Mehmed the Conqueror, but Murad certainly did his share to make it happen. His armies besieged both Constantinople and Thessaloniki, and while Constantinople held, it wasn't in any position to respond to the other city's desperate pleas for assistance. Thessaloniki was at this point ruled autonomously by a Byzantine royal cousin, Andronikos Palaiologos, whose grip on power began to slip as the people--aware that their emperor wasn't going to send any help--started demanding that he surrender to the Ottomans. In lieu of surrendering, though, Andronikos decided to try something else, and so he sold the city to Venice.
The Venetians were an eager buyer, as they were looking to expand and to take a more aggressive stance against the Ottomans in the eastern Mediterranean. Plus they had the money to make it worth Andronikos's while, so they were the perfect choice to purchase Thessaloniki and take over its defense. Emperor Manuel was asked to give his assent, and, really, what was he going to do? He was in no position to object. Venice also sent a messenger to the Ottomans, asking them to lift their siege and allow the Venetians to move into the city. That messenger was promptly thrown in one of Murad's prisons. I guess that was a "no." Murad seems to have felt that Thessaloniki was his by right, because it had been an Ottoman possession before Ankara and had only been returned to the Byzantines by an illegitimate pretender to the Ottoman throne, and in fact the Byzantine historian Doukas has him say as much to Venetian envoys.
The Ottomans besieged Thessaloniki for the next ~6 years, while the Venetians, who knew they couldn't win a pitched battle and really couldn't even do much to impede Ottoman supply lines, tried to negotiate terms under which they could assume control over the city in return for some kind of tribute payment to the Ottomans. In addition, they tried to harass the Ottomans asymmetrically, by sending fleets to strike periodically at Gallipoli (where Ottoman reinforcements and supplies were vulnerable as they crossed the Dardanelles), by trying to bribe/cajole other principalities in the Balkans/eastern Mediterranean to fight the Ottomans for them, and by trying to rally support in Catholic Europe for a Crusade. This last part never came to fruition (the Venetians were more interested in warring with Catholic Hungary than joining forces with them), and the other bits just weren't enough to dislodge the Ottoman siege. Efforts to get even small numbers of Venetian soldiers and supplies into the city to help support the defense generally were unsuccessful.
By the late 1420s, the people inside Thessaloniki--who never really had any input on being sold to Venice, remember--were starving. Soldiers began defecting to the Ottomans while on guard duty, just so they'd get to eat again. The frequent messages from Greek leaders of the city to Venice grew angrier, seeing that Murad was content to wait until they all starved to death and simply walk in and take the city, and that all these Venetian efforts to negotiate a deal with him were completely futile. They urged the Venetians to surrender the city, which was inevitably going to fall to the Ottomans anyway, before everybody inside was dead. The Venetians, we're told, responded to these missives by recruiting a force of soldiers inside the city whose job was to kill anybody caught advocating for surrender. Classy!
In late March 1430, Murad assembled his army before the walls of Thessaloniki and demanded its surrender. Amazingly, no surrender was forthcoming, so Murad (who was apparently now tired of waiting) made plans to storm the walls. The assault began on the morning of March 29, and within a few hours the depleted city was in Ottoman hands. The Venetians quickly cut a peace deal with the now very amenable Murad. Thessaloniki was heavily damaged by the days of looting that followed, by a conscious imperial effort to strip the city's wealth and bring it back to Edirne, and, well, by the siege itself, which left only a few thousand people alive when it finally ended, many of whom were sold off into slavery. After the initial wave of destruction passed, though, Murad had the city rebuilt and repopulated, and it remained one of the major cities of the empire until it was captured by Greece during the First Balkan War in 1912.