In the mean time, Maha Nawrahta opened the southern front on 23 October 1765  in three directions. He had 20,000 to 30,000 under his command.  A small army invaded by the Three Pagodas Pass towards Suphanburi. Another small army invaded down the Tenasserim coast towards Mergui  and Tenasserim  town. However, the main thrust of his attack was at Kanchanaburi. His 20,000-strong main southern army invaded via the Myitta Pass.  Kanchanaburi fell with little resistance. The main reason for the quick fall of Kanchanaburi could be that the Burmese were more battle-hardened. But it could also be that the Siamese command miscalculated where the Burmese main attack would come from, and had not sufficiently reinforced the fort to withstand a major attack. Judging by the Siamese chronicles' reporting of the main attack route, the Siamese command appeared to have believed that the main Burmese attack would come from the Gulf of Siam coastline, instead of the most obvious and shortest route via Kanchanaburi. The Siamese sources say that Maha Nawrahta's main invasion route came from southern Tenasserim, crossing the Tenasserim range at Chumphon and Phetburi. The path is totally different from the Kanchanaburi route reported by the Burmese chronicles. Historian Kyaw Thet specifically adds that the main attack route was via the Myitta Pass.

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