The Long March was an epic retreat by the three Red Armies of China between 1934 and 1936, which took place during the Chinese Civil War.
The Chinese Communist forces were partially surrounded and trapped by the Nationalist Kuomintang, led by Chiang Kai-shek. Threatened with extermination by the better-led and more numerous Kuomintang, the Communists fled west and north.
From their base in Jiangxi Province, southern China, the Red Armies set out in October of 1934, and according to Mao, marched some 12,500 kilometers (about 8,000 miles). More recent estimates from Ed Jocelyn and Andrew McEwen's book The Long March put the distance at a much shorter but still impressive 6,000 km (3,700 miles). This estimate is based on measurements the two Britons made while retracing the route - a large arc that ended in Shaanxi Province.
Modern Chinese Communist mythology celebrates the Long March as a great victory, and it did preserve the Red Armies from complete annihilation. The Long March also solidified Mao Zedong's position as the leader of the Communist forces.
However, the retreat was disastrous in terms of human losses and suffering. The Red Armies set out from Jiangxi with an estimated 85,000 troops. A mere 7,000 made it to Shaanxi - less than 1 in 10. (Some unknown amount of the reduction in forces was due to desertions, rather than deaths.)
Those who made it to the end of the march overcame rugged terrain and sub-zero temperatures, dangerous river crossings, attacks by the forces of various warlords and of course attacks by the Kuomintang.
Once it was ensconced in the relative safety of the north, the combined Red Army was able to recover and rebuild itself, finally defeating the Nationalist forces more than a decade later, in 1949.