By far, the most destructive aftershock of World War I was World War II, coming a mere 20 years after the Treaty of Versailles. While the rise of the Nazis in Germany in the 1930's generally took center stage, events elsewhere, some of them as far away as East Asia, also contributed to the outbreak of war. Three main factors, all resulting from World War I, would lead to war: the Treaty of Versailles, the Great Depression, and the Russian Revolution.
Along with leading to the rise of the Nazis, the Treaty of Versailles had quite different results on France's and Britain's relations with Germany and each other. Since they shared a long land border with Germany and had suffered a great deal in the war, the French were much more nervous about a resurgent Germany and wanted to keep its power limited. Therefore, in 1935, when Hitler announced that Germany would rearm (they had been doing so secretly for two years), France signed a series of defensive pacts with Germany's neighbors to contain any future aggression by Hitler. Among these pacts was one with the Soviet Union, which France saw as the primary counterweight to German power.
Britain, however, feared Stalin as much as it did Hitler, and signed a naval pact with Germany giving it the right to build a surface fleet 35% as big as Britain's and a submarine fleet as large as Britain's. While Britain apparently did not feel threatened by this, France did. Consequently, the two powers rarely cooperated effectively during the series of crises that occurred in the late 1930s, providing just the sort of disunity and lack of cooperation Hitler wanted.
Aggravating the situation was a sort of shell shock among the British and French caused by the horrible memories of World War I. Just as they had been too eager to go to war in 1914, now they were overly cautious and willing to appease aggressors in order to avoid a war. Unfortunately, dictators such as Hitler thrived on such weakness. Just as the lesson of 1914 was that too much aggression can lead to war, the lesson of 1939 would be that war can just as easily result from appeasement and giving in to aggression.
The Depression also had unsettling effects outside of Germany. Among other things, it seriously hurt Japan, whose economy depended heavily upon trade to pay for resources and food for its burgeoning population. As tariffs went up and the Depression deepened, Japan grew desperate for resources. This desperation led to a military takeover of the government, somewhat reminiscent of the Fascist dictators in Europe. In 1931, the Japanese seized Manchuria from China on the flimsy pretext of setting up the "independent" state of Manchukuo under Japanese "protection." China protested to the League of Nations, but the League had no power of its own to act against aggression, especially if that aggression were half a planet away. Therefore, Japan kept Manchuria and a foothold in China.
Even before this, China was already deeply mired in its own problems. European and Japanese aggression in the late 1800's had helped lead to turmoil in Chinese society and government. In 1912, a revolution replaced the last Chinese emperor with a republic under the western educated Sun Yat Sen. However, China's experiment in democracy floundered, and, after Sun Yat Sen's death, Chinese politics disintegrated into a three-way struggle for power between the Nationalist government's leader, Chiang Kai-shek, various independent warlords in the countryside, and the Communists led by Mao Zedong.
The Japanese seizure of Manchuria presented the Chinese government with a dilemma: fight Japan right away or crush the Communists and warlords first and then face the Japanese with a united front. Chiang Kai Shek, being strongly anti-Communist, decided to unify China first. For several years he waged intensive warfare against the Communists whom he badly damaged, but failed to destroy. However, Chiang's generals, anxious to turn against Japan, forced him to ally with Mao against the common enemy. Japan, fearing a united China, told the Nationalists to join it against the Communists or it would take "all the steps necessary to assure peace." In July 1937, it "assured" that peace by invading China.
The Chinese army was no match for the more mechanized Japanese forces, which relentlessly and brutally swept across the eastern seaboard of China. Cities were bombed and strafed mercilessly, while their populations were massacred with uncontrolled ferocity. Reeling from these losses, the Chinese switched to a strategy of trading space for time by retreating into the vast interior of China. This drew the advancing Japanese forces further and further inland and stretched their lines to the limit. The war now settled down to a costly stalemate that burnt, bled, and bent China, but could not break it.
As a result, the Japanese decided to look elsewhere for easier conquests. In 1939, they briefly turned north against the Soviet Union. However, defeat at the hands of Soviet forces in a short but sharply fought conflict plus a surprise pact by Japan's ally, Hitler, with Stalin to carve up Poland, convinced Japan to go elsewhere. Therefore, it turned to easier and more lucrative conquests in South East Asia. This involved attacking the colonies of France, Britain, and Holland, all of who were too preoccupied with the war then raging in Europe to effectively stop Japan.
This also brought Japan face to face with the United States. When the United States threatened economic sanctions against the Japanese if they did not pull back, Japan launched a surprise attack on the American naval base at Pearl Harbor in the Hawaiian Islands (12/7/1941). From the American perspective, this was the beginning of the Second World War in the Pacific, although the Chinese and others saw it as starting in 1937 with the Japanese invasion of China. Either way, the war in Asia was on.
Ironically, Japan's decision to turn south rather than north may have saved the allied cause in World War II. If Hitler had kept his Japanese allies informed on his intentions to attack Russia in 1941, they could have tied down enough Soviet forces in the Far East to deny Stalin vital reinforcements that would be a significant factor in the ultimate Russian victory against Germany. And, of course, a German victory against Russia would have seriously altered the course of World War II and subsequent history.
That leaves Russia, the other big power that should have been opposed to the Fascists. Unfortunately, relations with the Western powers were poisoned by bitterness over Allied intervention during the Russian Civil War and the deep ideological differences between capitalism and communism. As a result, there was no concerted action between Russia and the West against Fascist aggression. All these factors, the disunity between France and Britain, Russian hatred and distrust of the West, and the unchecked aggression of Japan in the East combined to expose the weakness and disunity of the former alliance against Germany.
As a result, the weakening of the old alliance triggered a vicious cycle of encouraging Fascist aggression which the Western democracies failed to react to, thus causing more aggression, and so on. This pattern was sadly played out several times in the 1930's before the West finally took its stand.
It started in 1935 when Hitler announced that Germany was going to rearm itself in defiance of the Treaty of Versailles. (Actually he had been secretly expanding German forces since 1933.) We have already seen how Hitler announced Germany's rearmament in 1935. Since he justified this with the principle of national self-determination, Britain and France did nothing to stop him. This merely encouraged more aggressive actions. Consequently, in 1935, Mussolini sent Italian forces into Ethiopia, using only the weakest of excuses to cover this blatant act of aggression. When the League of Nations threatened economic sanctions against Italy, Mussolini said a boycott on oil (which would have crippled his war machine) would mean war with the League's members. The League, without any real force to back it up, fell for this bluff. Britain wanted to stand up to Mussolini. However, France, still angry about Britain's naval pact with Germany and hoping to stay on good terms with Italy as a counterweight to growing German influence in Austria, refused to support Britain. As a result, Ethiopia fell as the world just stood by and watched.
Therefore, in 1936 Hitler defied the Treaty of Versailles again by moving German forces back into the Rhineland, the demilitarized part of Germany. This especially agitated France, who wanted British backing but received none. Since German rearmament was just starting, the German generals leading the troops into the Rhineland were under secret orders to turn back if they met any French resistance. They met no such resistance. Once again, Hitler got his way.
The aggression continued when the dictators, including Stalin got the opportunity to intervene in the Spanish Civil War. In 1931, unrest had led to the overthrow of the corrupt monarchy still ruling Spain. At first, a fairly liberal and democratic government took power. But, without a strong middle class and economy, riots and turmoil resurfaced. In 1936, the Fascist Phalangists, led by General Franco, seized power and started the Spanish Civil War.
Any civil war is a terrible thing, but Germany, Italy, and the Soviet Union chose to intervene and make the war in Spain much worse. Hitler and Mussolini backed the Fascists, known as the Nationalists. Stalin threw his support behind the Republicans, also known as Loyalists, who had many socialists and communists in their ranks. The result was a disaster for Spain, as terrorists from both sides murdered civilians and leaders from the opposition, and the German air force practiced the new tactics of aerial bombardment on Spanish towns.
The most famous of these atrocities, immortalized by the Spanish painter, Picasso, was the bombing of the Spanish town of Guernica, where over one-third of its population of 7000 were killed or maimed just because they were in the way. While that was a mere fraction of the millions that would die from aerial raids in the Second World War, it shocked the world since it was documented on film and also because it symbolized a sinister new turn in modern warfare. In the end, the Fascists won again as the Western democracies just watched from the sidelines. The question was: how much further could Fascist aggression go unchallenged? Hitler seemed determined to find out.
Hitler, further encouraged in his contempt for the Western democracies, next moved on to an even bolder objective: the Anschluss (unification) of Austria with Germany. Hitler, himself being of Austrian birth, claimed the Austrians were Germans whose drive to achieve national self-determination was being stifled by being kept separate from the rest of Germany. Whether right or wrong, this logic helped paralyze France and Britain into inaction once again. Therefore, Austria became part of Germany in 1938 whether the Austrians liked it or not.
The next target of Nazi aggression was the Sudetenland, a part of Czechoslovakia with a large German population along with much of the country's industry and defensive fortifications. Raising the cry of national self-determination once again, Hitler threatened war with anyone who got in his way. A conference between Britain, France, Italy, and Germany met at Munich where the Fascist dictators bullied and persuaded France and Britain to agree to the Nazi takeover of the Sudetenland. Convinced, or at least wanting to believe, that this was all Hitler wanted and that he also wanted peace, they gave in to him once more, without even consulting their Czech allies. They figured this was all Hitler wanted.
In March 1939, Hitler swallowed up the rest of Czechoslovakia without French or British resistance. This had two effects. For one thing, France and Britain were now finally convinced that Hitler would not stop on his own and were determined to stand up to him the next move he made. Unfortunately, at the same time, Stalin was convinced that France and Britain would do nothing to stop any further Nazi aggression in Eastern Europe. Therefore, he signed a pact with Hitler (August, 1939) that would carve up Poland between them.
On September 1, 1939, believing Britain and France would do nothing to stop him, Hitler invaded Poland. Two days later, France and Britain declared war on Germany. A mere twenty years after the end of the First World War, the Second World War had begun.