In the early 17th century, the Restored Toungoo kings required the vassal kings to reside at the capital for long periods, and abolished their militias and their fortified defences. When the Dutch merchants visited Burma in the mid-17th century right after the change was instituted, they were amazed that even the major towns except the capital did not have any fortified defences. They found that the Burmese kings distrusted the vassal states, and instead preferred to rely on the country's numerous toll stations and watchtowers from where messengers could be rushed to the capital. Despite the royal prohibition, the fortifications returned to the scene during the Burmese civil war of the 1750s, which featured a series of sieges by both sides. In the 19th century, forts along the Irrawaddy was a major part of Burmese strategy to defend against a potential British invasion. In practice, however, they did little to withstand British firepower. A major strategy of the army was the use of scorched earth tactics, mainly in times of retreat but also in times of advance. They would burn and destroy everything in sight that could be of use to the enemy, crops and infrastructure (wells, bridges, etc.). At times, the entire region at the border was destroyed and depopulated to create a buffer zone. For example, in 1527, King Mingyi Nyo depopulated and destroyed the infrastructure of the entire Kyaukse–Taungdwingyi corridor between Ava (Inwa) and his capital Toungoo (Taungoo). Likewise, the Burmese had left the entire Chiang Mai region depopulated and its infrastructure destroyed in the wake of their 1775–1776 war with Siam. The army also used scorched earth tactics as a means to intimidate the enemy and to secure easier future victories. The ruthless sacks of Martaban in 1541 and Prome in 1542 served to secure the fealty of Lower Burma to the upstart regime of Tabinshwehti of Toungoo. Threatened by Bayinnaung's army of a sack of their capital Ayutthaya, the Siamese duly surrendered in 1564. When the Siamese changed their mind in 1568, the city was brutally sacked in 1569. Two hundred years later, the army's brutal sack of Pegu in 1757 secured the subsequent tribute missions of Chiang Mai, Martaban and Tavoy to Alaungpaya's court.