Many communists such as Mao have viewed the national movement as a step towards national liberation. Then after that the march is on towards socialism and then communism. This has been particularly important in the Third World. Many communists have been more nationalist. Stalin successfully played on Russian nationalism in maintaining his regime.
Communism supports the people, and the people make up the nation. Communists are against the what they perceive to be sham nationalism propagated by capitalists and fascists.