The province of Kanchanaburi, touching the Myanmar border, is punctuated with memories from World War II. The most famous being the Bridge over the River Kwai, part of the Thai-Burma railway (aka Death Railway) built by prisoners of war.
There’s a memorial museum dedicated to those that lost their lives during the railway’s construction and a peaceful war cemetery.
You can take the train straight from Bangkok, and it’s possible to see the sights on a long day trip, but to immerse yourself in the region’s turbulent history there are plenty of places to stay in Kanchanaburi Town. A longer stay will reward you with tranquil hiking paths, and perhaps even exploring an ancient Khmer temple on the banks of the river Kwai.