The years after WW1 saw theorists looking at how this promising new military dimension might be deployed most effectively. ‘Theory and practice: the interwar years 1919–1939’ considers the new structures, the ideas that informed them, and technical developments, and how all were brought together in the Spanish Civil War. It describes how theorists thought air power could win wars alone, either by terrorizing citizens in order to force their governments to capitulate, or through demolishing a state’s industrial capability to sustain a war. This was, and is, termed ‘strategic bombing’. Most importantly, air power offered the possibility that future conflict might be cheaper than the ‘old’ wars, in both lives and money.