why does the poet want to call war 'wasteful war'?

Wars are wasteful because their occurance does not effect the due course of time. Here, the poet is trying to explain the power of time over man kind and the importance of words over monuments. He explains that people erect monuments for their memories to last forever, however, even they get destroyed with the passage of time. Thus, it is proven that nothing can surpass the power of words in the form of poems and ballads. Wars and battles can destroy everything which is tangible but the words would immortalise the person.