While the city withstood a siege by the Sassanids and Avars in 626, Heraclius campaigned deep into Persian territory and briefly restored the "status quo" in 628, when the Persians surrendered all their conquests. However, further sieges followed the Arab conquests, first from 674 to 678 and then in 717 to 718. The Theodosian Walls kept the city impregnable from the land, while a newly discovered incendiary substance known as Greek Fire allowed the Byzantine navy to destroy the Arab fleets and keep the city supplied. In the second siege, the second ruler of Bulgaria, Khan Tervel, rendered decisive help. He was called "Saviour of Europe".