The Lao People's Democratic Republic, along with North Korea, China, Cuba and Vietnam is one of the world's five remaining communist states. The only legal political party is the Lao People's Revolutionary Party(LPRP). Laos is the only landlocked country in Southeast Asia. Its thickly forested landscape consists mostly of rugged mountains, the highest of which is Phou Bia at 2,818 metres (9,245 ft), with some plains and plateaus. The Mekong River forms a large part of the western boundary with Thailand, whereas the mountains of the Annamite Range form most of the eastern border with Vietnam and the Luang Prabang Range the northwestern border with the Thai highlands. There are two plateaux, the Xiangkhoang in the north and the Bolaven Plateau at the southern end. The climate is tropical and influenced by the monsoon pattern. From 1964 to 1973, the U.S. dropped more than two million tons of ordnance on Laos during 580,000 bombing missions—equal to a planeload of bombs every 8 minutes, 24-hours a day, for 9 years – making Laos the most heavily bombed country per capita in history.