International alliances and agreements played a critical role in the lead-up to World War II. They were a double-edged sword; on one hand, they aimed to maintain peace and balance of power, but on the other, they sometimes inadvertently paved the way for conflict. The League of Nations, intended to prevent wars, proved ineffective due to its lack of enforcement power and the absence of key nations like the United States. The Munich Agreement and the policy of appeasement, meant to avert war, only emboldened aggressors like Nazi Germany. Conversely, mutual defence pacts, like the Anglo-Polish military alliance, were crucial in determining the responses to aggression, as they bound nations like Britain to defend Poland, leading to their entry into the war following Germany's invasion of Poland. The Nazi–Soviet Pact dramatically altered the European balance of power and directly led to the outbreak of World War II, showcasing how strategic alliances could rapidly change the geopolitical landscape.

The global economic situation in the 1930s, particularly the Great Depression, had a profound influence on international relations and the onset of World War II. Economic hardship led to political and social instability in many countries, creating fertile ground for extremist ideologies and regimes, notably in Germany, Italy, and Japan. These regimes pursued aggressive expansionist policies, partly driven by economic motivations such as the need for resources and markets. The economic strain also influenced the foreign policies of other countries. For example, the economic difficulties in Britain and France contributed to their initial reluctance to confront aggressors, as they were more focused on domestic economic recovery than on foreign entanglements. Additionally, the economic crisis undermined the effectiveness of international institutions like the League of Nations, as member states were more preoccupied with national economic issues than collective security. This overall economic context set the stage for the aggressive moves by totalitarian regimes, which ultimately led to the outbreak of World War II.

Britain's initial reluctance to rearm in the 1930s was due to a combination of factors. Primarily, the collective memory of the horrors and losses of World War I fostered a strong aversion to military conflict and a hope for sustained peace. Additionally, the economic constraints of the Great Depression made significant military expenditure unpalatable. The British government, under leaders like Neville Chamberlain, also harboured a belief that diplomatic engagement and appeasement could resolve conflicts with aggressive powers like Nazi Germany. This view was underpinned by a widespread public sentiment against war. Furthermore, there was a certain level of underestimation of the Nazi threat, with a belief that Germany was still recovering from its post-World War I challenges and not an immediate danger.

British public opinion in the late 1930s significantly influenced the government's foreign policy. The trauma of World War I had instilled a deep-seated desire for peace among the British populace, leading to widespread support for disarmament and appeasement. This sentiment was reflected in the government’s initial reluctance to confront Nazi Germany, despite its aggressive policies. Prime Minister Neville Chamberlain’s policy of appeasement towards Hitler, including the Munich Agreement, was largely a response to this public desire to avoid war at almost any cost. However, as the true nature of Hitler's ambitions became apparent, especially after the invasion of Czechoslovakia, public opinion began to shift towards supporting a firmer stance against aggression. This shift played a crucial role in Britain's eventual move away from appeasement and towards rearmament and military commitments to other nations, such as Poland.

The nature of warfare underwent significant changes between World War I and World War II, impacting British military strategy. World War I was characterised by trench warfare and static fronts, but by the late 1930s, military technology had evolved. The development of tanks, aircraft, and mechanised infantry led to the concept of blitzkrieg or lightning war, which Germany effectively utilised. This new form of warfare emphasised speed, surprise, and combined arms tactics, making traditional defensive strategies less effective. In response, Britain focused on modernising its armed forces, particularly the Royal Air Force, understanding the crucial role of air superiority. The emphasis was also on developing an effective radar system for early warning and on mechanising the army to improve mobility and response times. This shift in strategy was a response to the lessons learned from World War I and the evolving nature of military threats.