At its narrowest point, between the Thai cities of Kra Buri and Chumphon, the coastal distance between the Andaman Sea and the Gulf of Thailand is only 44 km (27 mi).

The southern part is the Phuket Range, which is a continuation of the Tenasserim Hills, extending further northwards for over 400 km (250 mi) beyond the Three Pagodas Pass.

Because of the International Date Line, this actually occurred hours before the 7 December (Hawaii time) attack on Pearl Harbor, making it the first major action of the Pacific War.

Japanese forces then moved south towards Perlis and Penang as part of the Malayan campaign, which culminated in the capture of Singapore.

The northernmost route was championed by Edward O'Riley (1821-1856), a government official in Burma, and Henry Wise, in England, when it was the subject of a report to the British Parliament in 1859 by Consul Robert Schomburk from Bangkok.