What happened to cause Chinese Communists to undertake the Long March in 1933

In 1934 the Communists suffered huge losses against the nationalists in the Chinese Civil War. Faced with the prospect of annhilation the leaders of the Comunist army shose to start what is now called the Long March. This was a tactic of withdrawal into mountainous areas, some 2500 miles to the north and west. The purpose was simple: survival. By undertaking a tactical withdrawal the Communists could ensure that they still had an effective fighting force; could select easy to defend locations; could regroup and could ensure that the Nationalists supply lines were long and susceptible to attacks.