Complex sentence: During the Ayutthaya period in Thai history (14th-18th centuries), the pass was the main invasion route for the Burmese, but was also occasionally used in the opposite direction by Thai armies.
Split into simpler sentences: During the Ayutthaya period in Thai history (14th-18th centuries), the pass was the main invasion route for the Burmese. It was also occasionally used in the opposite direction by Thai armies.