The United States, concerned with events in Europe and Asia, passed the Neutrality Act in August of the same year. A month later, Germany and Japan signed the Anti-Comintern Pact, which Italy joined the following year. Japan withdrew from the League of Nations after being condemned for its incursion into Manchuria. China appealed to the League of Nations to stop the Japanese invasion of Manchuria. Asia, staged the Mukden incident as a pretext to invade Manchuria and establish the puppet state of Manchukuo. In October 1936, Germany and Italy formed the Rome-Berlin Axis. Hitler defied the Versailles and Locarno Treaties by remilitarising the Rhineland in March 1936, encountering little opposition due to the policy of appeasement.

From late summer to early autumn, Italy conquered British Somaliland and made an incursion into British-held Egypt. In October, Italy attacked Greece, but the attack was repulsed with heavy Italian casualties; the campaign ended within months with minor territorial changes. To assist Italy and prevent Britain from gaining a foothold, Germany prepared to invade the Balkans, which would threaten Romanian oil fields and strike against British dominance of the Mediterranean.

The Soviets demanded a second front.

All agreed that defeating Germany was the primary objective. The British argued that military operations should target peripheral areas to wear out German strength, leading to increasing demoralisation, and bolstering resistance forces; Germany itself would be subject to a heavy bombing campaign. An offensive against Germany would then be launched primarily by Allied armour, without using large-scale armies. At the Casablanca Conference in early 1943, the Allies reiterated the statements issued in the 1942 Declaration and demanded the unconditional surrender of their enemies. The Americans favoured a straightforward, large-scale attack on Germany through France. The Soviets demanded a second front. Eventually, the British persuaded the Americans that a landing in France was infeasible in 1942 and they should instead focus on driving the Axis out of North Africa.

Total and unconditional surrender in Europe was signed on 7 and 8 May, to be effective by the end of 8 May. German Army Group Centre resisted in Prague until 11 May. On 23 May all remaining members of the German government were arrested by the Allied Forces in Flensburg, while on 5 June all German political and military institutions were transferred under the control of the Allies through the Berlin Declaration.

The portable machine gun spread, a notable example being the German MG 34, and various submachine guns which were suited to close combat in urban and jungle settings. Development of SIGINT (signals intelligence). Cryptanalysis enabled the countering process of decryption. Most major belligerents attempted to solve the problems of complexity and security involved in using large codebooks for cryptography by designing ciphering machines, the most well-known being the German Enigma machine. The assault rifle, a late war development incorporating many features of the rifle and submachine gun, became the standard post-war infantry weapon for most armed forces.