"Or, a vision in a dream. A fragment...."See in text(Kubla Khan)
The poem’s subtitle foreshadows the theme of art as an endless pursuit of perfect expression. Coleridge calls the poem “a fragment” in a recognition of its limits. The reference to the “vision in a dream” refers both the the circumstances of Coleridge’s composition of the poem as well as the structure of the poem’s narrative: a dreamlike description of Xanadu is followed by a wakeful reflection.
It is interesting that the ocean is "lifeless." It may be supposed that "lifeless ocean" foreshadows what is to come from the events seen by "voices prophesying war!"