In this column:
http://www.straightdope.com/classics/a2_437.html
Cecil neglects one pertinent fact: from 1519 to 1659, Hapsburg Spain was almost constantly at war – with France, with the Netherlands, with the Ottoman Turks, with Protestant Germany, with whomever. This of course was a significant drain on resources.
I learn this from Paul Kennedy’s “The Rise and Fall of the Great Powers,” which Cecil recommends here: