What does it mean to fight in pieces during a tunnel war

Fought in pieces during the tunnel war. Tunnel Warfare Connected Into Pieces is the lyrics of the song "Tunnel Warfare", meaning in the course of tunnel warfare, fought into a piece. The song "tunnel war", alias "the theme song of the 1965 movie "tunnel war", is sung by the Song and Dance Troupe of the China Railway Cultural Workers' Troupe and the Opera Troupe of the Central Opera and Dance Drama Theater.