In 680 the Byzantine Emperor Constantine IV, having recently defeated the Arabs, led an expedition at the head of a huge army and fleet to drive off the Bulgars but suffered a disastrous defeat at the hands of Asparuh at Onglos, a swampy region in or around the Danube Delta where the Bulgars had set a fortified camp.

The Battle of Ongal took place in the summer of 680 in the Ongal area, an unspecified location in and around the Danube delta near the Peuce Island, present-day Tulcea County, Romania. It was fought between the Bulgars, who had recently invaded the Balkans, and the Byzantine Empire, which ultimately lost the battle. The battle was crucial for the creation of the First Bulgarian Empire.