On 3 September 1939 Britain declared war on Nazi Germany in response to the invasion of Poland and to Britain's belief that Hitler was seeking continental hegemony. That realization had begun to dawn on Britain's leadership in March 1939 with the German occupation of Prague in flagrant disregard of the Munich agreement. The belated response reflected the terrible shadow cast by the First World War – a shadow that persuaded British politicians that almost any alternative was better than war. The memory of that war, of the Somme, of Ypres, and of Passchendaele, persisted through the coming conflict and played an important role in the formulation and execution of British strategy, operations, and tactics in the Second World War.
The most direct impact of political attitudes that regarded war as unthinkable was a gross underfunding of Britain's defenses beginning in the 1920s and lasting well into the 1930s. Admittedly, the low level of defense spending resulted from serious economic problems as well as from an underestimation of the German danger. Whatever the cause, the impact was serious. The RAF suffered least, because the Chamberlain government forced the air staff to buy the less costly air defense program rather than that service's desired bomber fleet. Strategic factors played little role in the government's decision, but the support rendered Fighter Command enabled it to win the Battle of Britain. The navy's financial difficulties made a serious strategic situation (with commitments in Far Eastern, Mediterranean, and European waters) worse.