Task: Question Understanding
Input (English): Passage: In 1740, the Mon of Lower Burma revolted against the Toungoo Dynasty and founded the Restored Hanthawaddy Kingdom based in Pegu . Throughout the 1740s, the Hanthawaddy forces were winning against Upper Burma-based Toungoo armies. The Siamese were concerned by another rising power in Burma since a strong Burma historically meant future invasions to Siam.  Concerned, the Siamese court readily gave protection to the Burmese governors of Martaban  and Tavoy who had fled to Siam. In 1745, they sent a diplomatic mission to Ava  to evaluate the political situation there, and were received by the Burmese king Mahadhammaraza Dipadi. They saw an Ava court that was on its last legs. By 1751, Restored Hanthawaddy forces were closing in on Ava. The Siamese concerns about the emergence of another strong dynasty based in Pegu appeared imminent. Perhaps as a precautionary measure, the Siamese decided to move their forward base into the upper coast in 1751.  While it remains unclear as to whether the Siamese ever intended  to go beyond the coast into mainland Lower Burma, the Siamese action nonetheless rang alarm bells in Pegu. Deeply concerned, the Hanthawaddy leadership withdrew two-thirds of their army back down to Lower Burma immediately after they had toppled the last Toungoo king in April 1752. This redeployment of Hanthawaddy troops proved a critical turning point in Burmese history as it gave nascent Upper Burmese resistance groups much needed breathing room. The Hanthawaddy command left less than 10,000 men to pacify all of Upper Burma.  Taking advantage of light Hanthawaddy troops, one resistance group, the Konbaung Dynasty led by Alaungpaya, drove out Hanthawaddy troops from Upper Burma by May 1754. Konbaung armies invaded Lower Burma in 1755, and captured Pegu in 1757, ending the 17-year-old Mon kingdom.
Question: When did the Hanthawaddi withdraw most of their troops from Upper Burma?
Output (English): date