Task: Question Understanding
Input (English): Passage: Concerned by the Burmese consolidation of Shan states and Laotian states, China launched four invasions of Burma between 1765 and 1769. At first, the Qianlong Emperor envisaged an easy war, and sent in only the Green Standard troops stationed in Yunnan. The Qing invasion came as the majority of Burmese forces were deployed in their latest invasion of Siam. Nonetheless, battle-hardened Burmese troops defeated the first two invasions of 1765-1766 and 1766-1767 at the border. The regional conflict now escalated to a major war that involved military manoeuvres nationwide in both countries. The third invasion  led by the elite Manchu Bannermen nearly succeeded, penetrating deep into central Burma within a few days' march from Ava. But the Bannermen of northern China could not cope with unfamiliar tropical terrains and lethal endemic diseases, and were driven back with heavy losses. After the close-call, Hsinbyushin redeployed his armies from Siam to the Chinese front. The fourth and largest invasion got bogged down at the frontier. With the Qing forces completely encircled, a truce was reached between the field commanders of the two sides in December 1769.
Question: When did the last invasion happen?
Output (English): date