World War II broke out when Nazi Germany invaded Poland on September 1, 1939. A once preventable war had become inevitable -- and would soon become global -- due to three fatal decisions.
Most infamously, the Western European democracies had appeased Hitler during the late 1930s in hopes that he would quit gobbling up his neighbors. Unfortunately, the Nazis considered Western appeasement as weakness to be manipulated rather than magnanimity to be reciprocated.
After the bloodless annexation of Austria and the dismemberment of Czechoslovakia, Hitler assumed that Britain and France would not go to war at all if he went into Poland. Or, if they did, that they would not fight very seriously.
Yet Western appeasement did not alone guarantee the outbreak of World War II.
The Germans invaded Poland only after a guarantee from Josef Stalin that the Soviet Union would soon join in attacking the Poles from the east. The two dictatorships could then divvy up the country.
Stalin's communist Russia had foolishly gambled that by making a deal with Nazi Germany, Hitler would leave the Soviets alone. At first, Stalin hoped that Germany would turn its war machine loose only on the Western European democracies.
Yet Stalin's collaboration with Hitler eventually guaranteed that Russia also would be double-crossed -- less than two years after signing an agreement with the Third Reich. Germany surprise-attacked the Soviet Union on June 22, 1941. Due to Stalin's collaboration, almost 30 million Russians would die on the Eastern Front over the next four years.
But it was more than Western appeasement of Hitler and Soviet collaboration that made World War II inevitable. Nazi Germany still remained relatively weak in 1939. The populations, economies and territories of its likely enemies were collectively far greater than those of the Third Reich and its allies.
A third, fatal decision was necessary to ensure a war. The United States had entered World War I late in April 1917, and it revived the sagging Allied effort, helping to crush the Germany army and win the war by November 1918.
But by 1919, America had rapidly disarmed and forgotten its key role in World War I. Americans had tired of the Europeans. They were sick of the endless horse-trading that had led to the postwar Versailles Treaty.
By the start of the Great Depression in 1929, America was mostly unarmed and determined never to get involved in European feuding again. Most Americans complained that the huge death toll of World War I had led to neither perpetual peace nor even a peaceful Germany.
America's isolationism and disarmament also helped prompt another global war. Had the U.S. kept its military strong after World War I, and had it entered into a formal alliance with its former World War I partners, Germany never would have risked a second war against the combined strength of a fully armed Britain, France and United States.
Instead, Hitler assumed the U.S. either could not or would not offer much military help to his intended European targets.
Why, then, did a relatively weak Nazi Germany between 1939 and 1941 believe that it could take on much of the world, and inspire Axis partners such as Italy and Japan to follow its suicidal lead?
The answer is obvious. British and French appeasement, Soviet collaboration and American isolation had together convinced Hitler and his Axis allies that the victors of World War I were more eager to grant concessions at any cost than were the defeated.
The world of 2016 is eerily beginning to resemble the powder keg of 1939 Europe.
Iran, China and North Korea, along with radical Islamic terrorist groups, all have particular contempt for Western democracies. Almost daily, various aggressive nations or organizations seek provocation by shooting off intercontinental missiles, boarding American boats, sending millions of young male Middle Easterners into the West, and issuing unending threats. China is creating new artificial islands to control commercial routes to and from Asia.
The European Union is largely unarmed. Yet it still trusts that it can use its vaunted "smart diplomacy" to reason with its enemies.
Meanwhile, Vladimir Putin's Russia cuts deals with Iran, Syria and most of the enemies of the West. Like Stalin before, Putin cynically assumes that his triangulations will turn aggressive powers exclusively against the West. Recently, he warned the West of a "new world war" starting in the Middle East.
America is slowly withdrawing from involvement abroad, using the same isolationist arguments heard in the 1920s.
Past interventions in the Middle East have worn on the nation. Ingrate nations did not appreciate American sacrifices. In tough economic times, some contend that defense spending should be diverted to more social programs.
Appeasement, collaboration and isolationism always prove a lethal mix -- past and present.
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Victor Davis Hanson, a classicist and military historian, is a senior fellow at the Hoover Institution and a recipient of the 2007 National Humanities Medal.