The English tactic of the Chevauchee during the Hundred Years War was exactly that kind of economic warfare. By devastating swathes of French territory, reducing the income of the King of France through taxation, the English reduced the French ability to raise or pay armies and made French resistance to English conquest so much harder. An accessible on-line source is here http://europeanhistory.about.com/od/thehundredyearswar/fl/The-Chevaucheacutee.htm and is discussed in more detail here http://ebooks.cambridge.org/ebook.jsf?bid=CBO9781139167789 The "Harrying of the North" by William was a similar campaign of devastation to render the North of England incapable of supporting an army that might have come from Scandinavia to challenge the Normans. Medieval Italian armies often included "guastatori" (spelling!?) described as "devastators" whose role was to destroy vines and olive trees as part of the same kind of economic warfare. Described in Mallet's "Mercenaries and their Masters" https://books.google.co.uk/books/about/Mercenaries_and_Their_Masters.html?id=VMQqAQAAMAAJ&redir_esc=y