For ten points, name this geographic feature that links mainland Thailand to the Malay Peninsula.
ThesecondlargestcityonthisgeographicfeaturewasthecapitalandnamesakeoftheNegaraSriDharmaraja Kingdom, also known (^)  as the Kingdom of Ligor. The BRN, or National Revolutionary Front, is fighting a long-running insurgency on this feature's Songkhla province. The Sultanate of (+) Pattani was located on this geographic feature. The decline of trade routes over this geographic feature may have been responsible for the Kingdom of (*)  Funan's collapse, and the resort island of Ko Phuket is located off the coast of this geographic feature. For ten points, name this geographic feature that links mainland Thailand to the Malay Peninsula.
ThesecondlargestcityonthisgeographicfeaturewasthecapitalandnamesakeoftheNegaraSriDharmaraja Kingdom, also known (^)  as the Kingdom of Ligor.
Isthmus  of  Kra  (accept  Ko Khot Kra ), prompt on Malay Peninsula