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The Mekong River
Andes Mountains.
Colorado River
This would be China and Russia to the southeast. The feature that separates these two countries is the Amur River.
The Yalu River is the border of China and North Korea.
The distance is about 2,520 air miles from border to border.
The Mekong River forms a border between Thailand and Laos.
The physical feature that lies between the Muslim Empire's Northern border is the Caucasus Mountains. These mountains stretch between the Black Sea and the Caspian Sea, forming a natural barrier between the Muslim Empire and the surrounding regions to the north.
The Gobi desert
The border between Mexico and Texas is the Rio Grande River, also known as the Rio Bravo. The border between Arizona and the Mexican state of Baja California is the Colorado River (Rio Colorado), which also flows through the Grand Canyon and the Hoover Dam. The rest of the border between the United States of America and Mexico is made up of arbitrary straight lines, not corresponding to any natural feature.
The border line between Myanmar and Bangladesh is called the India-Myanmar-Thailand Trilateral Highway. The border line between Myanmar and India is known as the India-Myanmar border.
Vietnamese border raids in Thailand happened in 1979.