Together with his father and his father-in-law Basarab of Wallachia, Ivan Alexander fought in the Battle of Velbazhd against the Serbs at modern-day Kyustendil in 1330, in which Bulgaria suffered defeat.

[5][6][10] At about the same time, Belaur, a brother of Michael Asen III, rebelled in Vidin, probably in support of his deposed nephew Ivan Stefan's claim to the throne.

The Byzantines overran Bulgarian-controlled northeastern Thrace, but Ivan Alexander rushed southward with a small army and swiftly caught up with Andronikos III at Rusokastro.

[11]After giving the impression that he wished to negotiate, Ivan Alexander, reinforced by Mongol cavalry, overwhelmed the smaller but better organized Byzantine army in the Battle of Rusokastro.

To seal the alliance, he betrothed his eldest son, Michael Asen IV, to Andronikos's daughter Maria (Eirene), the marriage eventually taking place in 1339.

Ivan Alexander demanded the extradition of his cousin Šišman, one of the sons of Michael Asen III, threatening the Byzantine government with war.

Ivan Alexander's show of force backfired, as the Byzantines managed to see through his intentions and sent against him the fleet of their ally, the Turkish emir of Smyrna Umur Beg.

As the price for Ivan Alexander's support, the regency for John V Palaiologos ceded him the city of Philippopolis (Plovdiv) and nine important fortresses in the Rhodope Mountains in 1344.

[citation needed] During the same period, the Serbian king took advantage of the Byzantine civil war to take possession of what is now Macedonia, and of most of Albania and northern Greece.

To cement the treaty, Ivan Alexander's daughter Keraca Marija[26] was married off to the future Byzantine Emperor Andronikos IV Palaiologos,[4] but the alliance failed to produce concrete results.

[28] From the middle of the 14th century, Bulgaria fell prey to the aspirations of the Angevin king Louis I of Hungary, who annexed Moldavia in 1352 and established a vassal principality there, before conquering Vidin in 1365,[6][23] and taking Ivan Sratsimir and his family into captivity.

This stance backfired, as another Byzantine ally, Count Amadeus VI of Savoy, leading the Savoyard crusade, captured several Bulgarian maritime cities in retaliation, including Ankhialos (Pomorie) and Mesembria (Nesebǎr), though he failed to take Varna.

[6] The Bulgarian emperor used this sum and territorial concessions to induce his at least de jure vassals Dobrotica of Dobruja[30] and Vladislav I of Wallachia[31][32] to reconquer Vidin from the Hungarians.

To make matters worse, in 1369 (the date is disputed), the Ottoman Turks under Murad I conquered Adrianople (in 1363) and made it the effective capital of their expanding state.

[44] In 1353, Ivan Alexander issued a charter allowing Venetian merchants to buy and sell goods throughout Bulgaria after Doge Andrea Dandolo assured him they would observe the prior treaties between the two countries.

[citation needed] A piece of a garment signed by Ivan Alexander and interwoven with gold was discovered in a noble's grave near Pirot in the 1970s; today it is preserved in the National Museum of Serbia in Belgrade.

It is the first find of its kind, demonstrating a medieval tradition attested in writing according to which Orthodox rulers would present their most eminent dignitaries with a piece of a garment they had worn.