Tong Yabghu Qaghan, who becomes the new ruler (khagan) of the Western Turkic Khaganate in 618,maintains close relations with the Tang Dynasty of China, and may have married into the Imperial family.

Tong Yabghu's empire fought with the Sassanids of Iran.

In the early 620's, the khagan's nephew Böri Shad leads a series of raids across the Caucasus Mountains into Persian territory.

Many scholars have identified Tong Yabghu as the Ziebel mentioned in Byzantine sources as having campaigned (as khagan of the Khazars) with the Emperor Heraclius in the Caucasus against the Sassanid Persian Empire in 627-628.

A few scholars, including Chavannes, Uchida, Gao and Xue Zhongzeng assert that Tong Yabghu cannot be positively identified with Ziebel (or any Khazar ruler) and may actually have died as early as 626.

These scholars point to discrepancies in the dates between Byzantine and Chinese sources and argue that definitively conflating Ziebel with Tong Yabghu is an exaggeration of the extant evidence.

Following the First Siege of Constantinople by the Avars and Persians, the beleaguered Emperor Heraclius had found himself politically isolated.

He cannot rely on the Christian Armenian potentates of Transcaucasia, since they have been branded as heretics by the Orthodox Church, and even the king of Iberia prefers to befriend the religiously tolerant Persians.

Against this dismal background, he finds a natural ally in Tong Yabghu.

Earlier, in 568, the Turks under Istämi had turned to Constantinople when their relations with Persia soured over commerce issues.

Heraclius dispatches to the steppes his emissary, named Andrew, who in 625 promises to the Khagan some "staggering riches" in return for military aid.

The khagan, on his part, is anxious to secure the Chinese-Contantinople trade along the Silk Route, which had been disrupted by the Persians in the aftermath of the Second Perso-Turkic War.

He sends word to the Emperor that "I shall take revenge on your enemies and will come with my valiant troops to your help".

A unit of one thousand horsemen fights their way through Persian Transcaucasia and delivers the Khagan's message to the imperial camp in Anatolia.