Task: Question Understanding
Input (English): Passage: In the south too, the Mon-speaking kingdom based out of Martaban had slowly come into its own. It formally broke away from its overlord Sukhothai in 1330. The southern kingdom's relationship with Sukhothai had always been an opportunistic arrangement from the beginning, designed to secure its rear in case of invasions from Upper Burma. In March 1298, just four years after he became a vassal of Sukhothai, when the Upper Burma ceased to be a threat, Wareru conveniently asked for and received recognition directly from the Mongol emperor as governor even though Sukhothai itself was already a vassal of the Mongols. Indeed, when Sukhothai grew weaker, Martaban seized the Tenasserim coast from Sukhothai in 1319. A decade later, Sukhothai tried to reassert control, reoccupying Tenasserim, and attacking Martaban in 1330. Martaban's defences held. It threw off any formal ties with Sukhothai. Nonetheless, the kingdom remained fragmented into three power centres: the Irrawaddy delta in the west, Pegu in the middle and Martaban in the southeast, each with its own chief, who pledged nominal allegiance to the high king. In 1363, Martaban revolted, and outlasted Pegu's armies. After six years, Pegu gave up. Martaban would be independent for another 20 years until 1389.
Question: In what year did Sukhothai become a vassel of the Mongols?
Output (English): date