In the aftermath of World War I, the defeated German Empire was made to sign the Treaty of Versailles. This restricted German military and territorial growth and required the payment of massive war reparations. Civil war in Russia led to the creation of the communist Soviet Union which soon fell under the control of Joseph Stalin. In Italy, Benito Mussolini seized power as a fascist dictator promising to create a "New Roman Empire".  The ruling Kuomintang party in China launched a unification campaign against rebelling warlords in the mid-1920s, but was soon embroiled in a civil war against its former Chinese communist allies. In 1931, an increasingly militaristic Japanese Empire, which had long sought influence in China as the first step of its right to rule Asia, used the Mukden Incident as justification to invade Manchuria; the two nations then fought several small conflicts until the Tanggu Truce in 1933.

German troops at the 1935 Nuremberg Rally in 1933, National Socialist Adolf Hitler became the leader of Germany and began a massive rearming campaign.  This worried France and the United Kingdom, who had lost much in the previous war, as well as Italy, which saw its territorial ambitions threatened by those of Germany.  To secure its alliance, the French allowed Italy a free hand in Ethiopia, which Italy desired to conquer. The situation was aggravated in early 1935 when the Saarland was legally reunited with Germany and Hitler repudiated the Treaty of Versailles, speeding up remilitarization and introducing conscription. Hoping to contain Germany, the United Kingdom, France and Italy formed the Stresa Front. The Soviet Union, also concerned due to Germany's goals of capturing vast areas of eastern Europe, concluded a treaty of mutual assistance with France.

These alliances did not amount to much. The Franco-Soviet pact, required to go through the League of Nations bureaucracy before taking effect, was essentially toothless and in June of 1935, the United Kingdom made an independent naval agreement with Germany easing prior restrictions. The isolationist United States, concerned with events in Europe and Asia, passed the Neutrality Act in August.  In October Italy invaded Ethiopia, but was soon politically isolated, with Germany the only major European nation supporting its aggression. Alliances shifted, with Italy revoking its objections to Germany's goal of making Austria a satellite state.

In March of 1936, Hitler remilitarized the Rhineland in direct violation of the Versailles and Locarno treaties, receiving little response from other European powers.  When the Spanish Civil War broke out in July, Hitler and Mussolini supported fascist Generalísimo Francisco Franco in his civil war against the Soviet-supported Spanish Republic. Both sides used the conflict to test new weapons and methods of warfare.

With tensions mounting, efforts to strengthen or consolidate power were made. In October, Germany and Italy formed the Rome-Berlin Axis and a month later Germany and Japan, each believing communism–and the Soviet Union in particular–to be a threat, signed the Anti-Comintern Pact, which Italy would join in the following year. In China, the Kuomintang and communist forces agreed on a ceasefire to present a united front to oppose Japan.

Course of the war

War breaks out

Japanese forces during the battle of WuhanIn mid-1937, following the Marco Polo Bridge Incident, Japan began a full invasion of China. The Soviets quickly lent support to China, effectively ending China's prior cooperation with Germany. Starting at Shanghai, the Japanese pushed Chinese forces back, capturing the capital Nanjing in December. In June of 1938 Chinese forces stalled the Japanese advance by flooding the Yellow River. Though this bought time to prepare their defenses at Wuhan, the city was still taken by October.  During this time, Japanese and Soviet forces engaged in a minor skirmish at Lake Khasan; in May of 1939, they became involved in a more serious border war.

In Europe, Germany and Italy were becoming bolder. In March 1938, Germany annexed Austria, again provoking little response from other European powers. Encouraged, Hitler began making claims on the Sudetenland; France and Britain conceded these for a promise of no further territorial demands.  Germany soon reneged, and in March 1939 fully occupied Czechoslovakia.

German and Soviet officers in Poland alarmed, and with Hitler making further demands on Danzig, France and Britain guaranteed their support for Polish independence; when Italy conquered Albania in April, the same guarantee was extended to Romania and Greece.   The Soviet Union also attempted to ally with France and Britain, but was rebuffed due to western suspicions about Soviet motives and capability.  Shortly after the Franco-British pledges to Poland, Germany and Italy formalized their own alliance with the Pact of Steel; following this, in a move that shocked all other major powers, Germany and the Soviet Union concluded a non-aggression pact, including a secret agreement to split Poland and eastern Europe between them.

By the start of September the Soviets had routed Japanese forces and the Germans invaded Poland. France, Britain and the Commonwealth declared war on Germany but lent little support other than a small French attack into the Saarland.  In mid-September, after signing an armistice with Japan, the Soviets launched their own invasion of Poland.   By early October, Poland had been divided between Germany and the Soviet Union. During the battle in Poland, Japan launched its first attack against Changsha, a strategically important Chinese city, but was repulsed by early October.

Axis advances

British and French soldiers taken prisoner in Northern France following the invasion of Poland, the Soviets began moving troops into the Baltics. Finnish resistance in late November led to a four-month war, ending with Finnish concessions.  France and the United Kingdom, treating the Soviet attack on Finland as tantamount to entering the war on the side of the Germans responded to the Soviet invasion by supporting its expulsion from the League of Nations.  Though China had the authority to veto such an action, it was unwilling to alienate itself from either the Western powers or the Soviet Union and instead abstained.   The Soviet Union was displeased by this course of action and as a result suspended all military aid to China.   By mid-1940, the Soviet Union's occupation of the Baltics was completed with the installation of pro-Soviet governments.

In Western Europe, British troops deployed to the Continent but neither Germany nor the Allies launched direct attacks on the other. In April, Germany invaded Denmark and Norway. Denmark immediately capitulated, and despite Allied support Norway was conquered within two months.  British discontent over the Norwegian campaign led to the replacement of Prime Minister Neville Chamberlain by Winston Churchill on May 10th.

On that same day, Germany invaded France and the Low Countries, making rapid progress using blitzkrieg tactics. By the end of the month the Netherlands and Belgium had been overrun and British troops were forced to evacuate the continent, abandoning their heavy equipment.  On June 10th, Italy invaded, declaring war on both France and the United Kingdom; twelve days later France surrendered and was soon divided into German and Italian occupation zones,[33] and an unoccupied rump state under the Vichy Regime. In early July, the British attacked the French fleet in Algeria to prevent their seizure by Germany.

With France neutralized, the Axis was emboldened. Germany began an air superiority campaign over Britain to prepare for an invasion and enjoyed success against an over-extended Royal Navy, using U-boats against British shipping in the Atlantic.  Italy began operations in the Mediterranean, initiating a siege of Malta in June, conquering British Somaliland in August, and making an incursion into British-held Egypt in early September. Japan increased its blockade of China in September by seizing several bases in the northern part of the now-isolated French Indochina.

Throughout this period, the neutral United States took measures to assist China and the Western Allies. In November 1939, the American Neutrality Act was amended to allow Cash and carry purchases by the Allies.  During 1940, the United States implemented a series of embargos, including oil, iron, steel and mechanical parts, against Japan; in September it agreed to a trade of American destroyers for British bases.

At the end of September the Tripartite Pact between Japan, Italy and Germany formalized the Axis Powers. As a warning to the United States, the pact stipulated that, with the exception of the Soviet Union, any country not currently in the war which attacked any Axis Power would be forced to go to war against all three.  The Soviet Union expressed interest in joining the Tripartite Pact, sending a modified draft to Germany in November and offering a very German-favourable economic deal; while Germany remained silent on the former, they accepted the latter.   Regardless of the pact, the United States continued to support the United Kingdom and China by introducing the Lend-Lease policy and creating a security zone spanning roughly half of the Atlantic Ocean where the United States Navy protected British convoys.

Soon after the pact, Italy's fortunes changed. In October, Italy invaded Greece but within days were repulsed and pushed back into Albania, where a stalemate soon occurred.  Shortly after this, in Africa, Commonwealth forces launched offensives against Libya and Italian East Africa. By early 1941, with Italian forces having been pushed back into Libya by the Commonwealth, Churchill ordered a dispatch of troops from Africa to bolster the Greeks. The Italian Navy also suffered significant defeats, with the Royal Navy puting three Italian battleships out of commission via carrier attack at Taranto, and several more warships neutralized at Cape Matapan.

The Germans soon intervened to assist Italy. Hitler sent German forces to Libya in February and by the end of March they had launched an offensive against the diminished Commonwealth forces. In under a month, Commonwealth forces were pushed back into Egypt with the exception of the besieged port of Tobruk. The Commonwealth attempted to dislodge Axis forces in May and again in June, but failed on both occasions. In early April the Germans similarly intervened in the Balkans, invading Greece and Yugoslavia; here too they made rapid progress, eventually forcing the Allies to evacuate after Germany conquered the Greek island of Crete by the end of May.

The Allies did have some successes during this time though. In the Middle East, Commonwealth forces first quashed a coup in Iraq which had been supported by German aircraft from bases within Vichy-controlled Syria, then, with the assistance of the Free French, invaded Syria and Lebanon to prevent further such occurrences.  In the Atlantic, the British scored a much needed public morale boost by sinking the German flagship Bismarck.  Perhaps most importantly, the Royal Air Force had successfully resisted the Luftwaffe's assault, and on May 11, 1941, Hitler called off the bombing campaign over Britain.

In Asia, in spite of several offensives by both sides, the war between China and Japan was stalemated by 1940. In August of that year, Chinese communists launched an offensive in Central China; in retaliation, Japan instituted harsh measures in occupied areas to reduce human and material resources for the communists.  Mounting tensions between Chinese communist and nationalist forces culminated in January 1941, effectively ending their co-operation.

With the situation in Europe and Asia relatively stable, Germany, Japan and the Soviet Union made preparations. With the Soviets weary of mounting tensions with Germany and the Japanese planning to take advantage of the European War by seizing resource-rich European possessions in Southeast Asia the two powers signed a neutrality agreement in April, 1941.  By contrast the Germans were steadily making preparations for an attack on the Soviet Union, amassing forces on the Soviet border, particularly in Finland and Romania.    next