Bengawan Solo (Solo River) was written by the kroncong singer Gesang Martohartono (above) in September 1940. A tribute to the beauty and significance of the river for the common people, the song subsequently assumed national importance, symbolizing the struggle for independence during the Japanese occupation of Java (1942–45).
The first widely popular song by an Indonesian composer written in Bahasa Indonesia, the Malay-based national language adopted by independent Indonesia, Bengawan Solo now evokes images of Indonesian revolutionary fighters to whom homage must be given. The song has spread throughout Southeast Asia, and it has even become popular in Japan and China, making it a potent symbol of pan-East/Southeast Asian identity.
This according to “The pan-East/Southeast Asian and national Indonesian song Bengawan Solo and its Javanese composer” by Margaret J. Kartomi (Yearbook for traditional music XXX  pp. 85–101).
Below, a recording featuring the voice of the composer.