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After occupying France, Germany prepared to invade Britain to force them out of the war. However, to cross the English Channel, they first had to neutralize the Royal Air Force. In the summer of 1940, the German Luftwaffe began attacking British airbases and radar installations, and the RAF fought back fiercely in defense. Britain responded to the Luftwaffe's attacks by utilizing its radar network and fighter control system. The German forces failed to completely subdue the RAF and subsequently expanded their bombing raids to major cities including London. British civilians endured large-scale air raids, but the government did not surrender. The British government, led by Winston Churchill, continued the war with support from the United States. Ultimately, Germany could not execute its invasion plan for mainland Britain and ceased the air campaign. The Battle of Britain became the first instance where Germany failed to achieve a decisive objective early in the war, and it served as a crucial turning point allowing Britain to continue fighting.

In the spring of 1940, Germany launched a large-scale offensive into Western Europe. German forces attacked the Netherlands, Belgium, and Luxembourg in succession, breaking through the Allied defense lines. France and Britain had anticipated the main German thrust through Belgium, but the German army instead advanced rapidly through the Ardennes Forest into northern France. German tank divisions broke through the Allied defenses and reached the English Channel, isolating British and French forces. The British conducted a massive evacuation operation from Dunkirk, moving approximately 330,000 troops back to Britain. France continued to resist but could not stop the German advance. Paris fell in June, and the French government signed an armistice with Germany. Northern and western France were occupied by German forces, while the Vichy regime was established in the south. Germany had seized control of key areas in Western Europe, leaving Britain as the only major European nation still opposing Germany.

On September 1, 1939, Germany invaded Poland. The German army rapidly broke through Polish defenses, heavily utilizing tanks and aircraft. The Soviet Union also entered Poland from the east on September 17. Poland found itself isolated, attacked from both Germany and the Soviet Union. Britain and France demanded Germany withdraw, but when their demand was rejected, they declared war on Germany on September 3. This marked the full-scale beginning of World War II in Europe. The Polish army resisted until the very end, but it was difficult to fend off simultaneous attacks from Germany and the Soviet Union. With the fall of Warsaw at the end of September, Poland's organized resistance effectively ceased. Subsequently, Germany and the Soviet Union divided and occupied Polish territory. Although Britain and France had declared war on Germany, no large-scale ground battles occurred on the Western Front for some time. This period was known as the Phoney War. However, Europe as a whole had already entered a prolonged conflict.

In 1938, Germany continued its territorial expansion by annexing Austria and demanding the Sudetenland region of Czechoslovakia. The UK and France negotiated with Germany to avoid war, agreeing in the Munich Agreement of September 1938 to cede the Sudetenland to Germany. However, Hitler did not stop there and occupied the Czech regions in March 1939. Germany then intensified its territorial demands on Poland and signed the Molotov-Ribbentrop Pact with the Soviet Union. This pact also included a secret agreement to divide Eastern European spheres of influence. Meanwhile, the cooperative relationship between Japan, Italy, and Germany strengthened, rapidly worsening the international situation. The UK and France declared their intention to respond to further German expansion by guaranteeing Poland's independence. Finally, on September 1, 1939, Germany invaded Poland, and Britain and France declared war on Germany, marking the beginning of World War II.

During the interwar period, authoritarian regimes and militarism strengthened in Germany, Italy, and Japan. Italy's Mussolini established a fascist regime and pursued foreign expansion, invading Ethiopia in 1935. In Germany, after Hitler became Chancellor in 1933, he solidified the Nazi dictatorship and pushed for military buildup and territorial expansion. In 1936, Germany and Italy strengthened their cooperation by supporting Franco's forces in the Spanish Civil War, and in the same year, they signed the Anti-Comintern Pact with Japan, expanding the anti-communist bloc's solidarity. Japan also expanded its aggression against China after the Manchurian Incident, starting the Second Sino-Japanese War in 1937. The League of Nations attempted sanctions against the aggressor nations but lacked the enforcement power to effectively stop them. Britain and France, also seeking to avoid a new war, failed to strongly curb the expansion of Germany, Italy, and Japan in the early stages.

The global economy seemed relatively stable in the late 1920s, but the situation changed drastically with the onset of the Great Depression in the United States in 1929. Triggered by the stock market crash in New York, finance and industry contracted, and unemployment surged. The downturn in the US economy quickly spread to Europe through trade and finance. Germany, heavily reliant on American loans, suffered a particularly severe economic blow, leading to widespread unemployment and poverty. As the economic crisis deepened, dissatisfaction with liberal democracy grew in various countries, and extremist political forces gained traction. In Germany, the Nazi Party, led by Adolf Hitler, expanded its support by exploiting public anxiety and nationalism. In Italy, Mussolini's Fascist regime had already seized power. The economic crisis weakened international cooperation and became a significant backdrop that led nations to strengthen their self-centered policies.

In the early 1920s, Europe strove to recover from the scars of war and economic turmoil. Germany faced severe economic hardship due to immense reparation burdens, and the crisis deepened in 1923 when France and Belgium occupied the Ruhr region over reparation issues. Hyperinflation, causing a drastic drop in currency value, occurred in Germany, inflicting significant damage on the middle class and laborers. Subsequently, with financial support from the US, the Dawes Plan was implemented, stabilizing the German economy and gradually improving relations among European nations. The Locarno Treaties in 1925 signaled an effort to guarantee Western European borders, and Germany joined the League of Nations in 1926. By the mid-1920s, international cooperation had strengthened compared to the immediate post-war period, seemingly establishing peace, yet the underlying conflicts were far from resolved.

After World War I ended, Europe strove to create a new world order. At the Paris Peace Conference in 1919, countries like the United States, Britain, and France discussed post-war settlements. Germany was subjected to territorial and military restrictions, as well as reparations, through the Treaty of Versailles. The dissolution of the Austro-Hungarian and Ottoman Empires also significantly altered the borders of Europe and the Middle East. The League of Nations was established to peacefully resolve disputes between nations, but the US did not join, and its enforcement power was limited. While new nations emerged, issues of ethnicity and territory remained. Europe's economy was weakened by the war's devastation, and the United States rapidly rose to become the center of the global economy. The post-war peace system outwardly pursued stability but failed to fully resolve the dissatisfaction of the defeated nations and ethnic conflicts.

World War I ended with Germany's armistice in 1918. For about four years, battles took place primarily in Europe and various regions around the world, claiming the lives of countless soldiers and civilians. During the war, machine guns, heavy artillery, tanks, aircraft, submarines, and chemical weapons were used, showcasing the characteristics of modern industrial warfare. After the war, the imperial orders of Germany, Austria-Hungary, the Ottoman Empire, and Russia collapsed, leading to the emergence of new nations. A new international order was established centered around the Treaty of Versailles and the League of Nations, but issues such as Germany's reparations, territorial disputes, and ethnic conflicts were not fully resolved. The Russian Revolution introduced new ideological confrontations into global politics, and the United States further expanded its economic and diplomatic influence in the international community. It also impacted nationalist movements in Asia, the Middle East, and Africa. World War I dismantled the 19th-century European-centric international order and shaped the new international political order of the 20th century, leading to World War II.

After World War I, the international community sought to establish a new order to prevent large-scale wars from occurring again. The concept of the League of Nations, proposed by U.S. President Woodrow Wilson, aimed to resolve disputes between nations through international cooperation. The League of Nations officially launched in 1920, with many countries participating as member states. However, the United States did not join the League of Nations as it failed to obtain ratification from the Senate. The war caused significant damage to Europe's economy and industrial facilities, and numerous soldiers and civilians died or were injured. The collapse of the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, and the Russian Empire dramatically altered the political landscape of Europe. In Russia, a communist regime emerged through revolution, and new nations were formed in Central and Eastern Europe. The war also impacted colonial regions, serving as a catalyst for the further spread of nationalism and independence movements. World War I was more than just a military conflict; it became a crucial historical turning point that transformed 20th-century international politics and social structures.