Here is a comprehensive summary of the Wikipedia article about Peuce Island:

Peuce was a significant ancient island in the Danube Delta, located in Scythia Minor (present-day Tulcea County, Romania), and was comparable in size to the island of Rhodes. In the 330s BC, the West Thracian Triballian king Syrmus took refuge there from Alexander the Great, who ultimately failed to capture the island due to its strong defenses and swift river currents. A portion of the Bastarnae tribe (a mixed Germanic-Celtic society) later settled on Peuce in the 2nd century BC, adopting "Peucini" as an alternative tribal name.

The island holds further historical importance as the presumed birthplace of Alaric I, the Visigoth king renowned for his 410 AD sack of Rome. It was also the site of the Battle of Ongal in 680 AD, a pivotal event that led to the creation of the First Bulgarian Empire. Peuce eventually disappeared during the Early Middle Ages due to changes in the Danube's distributaries.