It is misleading to think of 1939-1940 as the phoney war or twilight war, or the Sitz-krieg, or bore-war, with all the sense of lack of authenticity and reality these terms now convey. This was war as Britain and France wanted to wage it and win it. The dominant propaganda line in Britain in the early months of the war was ‘assurance of victory’, the policy was economic offence and military defence. Britain was fighting a war ‘for King, for Empire, and for Freedom’, as British propaganda put it. One reason we still think of the early war as phoney is that the air force was not used for bombing and that Germany was not attacked on land either. But the navy went on the offensive. German shipping and trade were swept from the seas. Imports into Germany were blocked, and from November 1939 its exports too, both extraordinary changes to a world economic system in which Germany was a more significant trader than France.
That is from the quite good Britain’s War Machine: Weapons, Resources, and Experts in the Second World War, by David Edgerton.