Politicians are not prophets. They make mistakes. Sometimes those mistakes are errors of optimism -- the Bush administration, for example, failed to gauge the level of resistance that would follow the overthrow of Saddam Hussein. Such mistakes often have dire consequences. No one can deny that the situation in Iraq would be better today if pessimistic realism had dominated the White House and Pentagon in March 2003.

But errors of optimism are far less dangerous and costly than failures of imagination. It was failure of imagination that led Neville Chamberlain to appease Hitler -- Chamberlain and his cronies simply could not conceive that a world leader could be so shockingly barbaric. It was failure of imagination that led American intelligence agencies to dramatically underestimate the capabilities of an Islamic terrorist group called al Qaeda during the 1990s. And it is failure of imagination that leads today's left to call for immediate withdrawal from Iraq and immediate mollification of Iran.

The left simply cannot understand the nature of our enemies in Iraq and Iran. They see Mahmoud Ahmadinejad as a misunderstood rhetorician, a blustering but good-natured man of peace. They see Iraq as a sordid civil war, a tribalistic swamp bound to remain mired in ethnic warfare for the foreseeable future. Democratic Senator and presidential candidate Barack Obama sums up the feelings of the mainstream anti-war left when he states, "We need to immediately begin the responsible removal of our troops from Iraq's civil war."

Ninety years ago, Western civilization removed its troops from another messy situation: the Russian Civil War. In 1917, Vladimir Lenin tore down the constitutional government of Russia. In early 1918, after he replaced it with a communist dictatorship and signed a peace treaty with the Germans, civil war broke out. Britain and America placed troops on the ground in Russia in an attempt to restore order and compel the Russians to reactivate the Eastern Front against Germany.

The Western effort quickly collapsed due to lack of motivation. British Prime Minister Lloyd George halfheartedly supported intervention in Russia, but quickly backed down after Germany surrendered. President Woodrow Wilson offered weak assistance to the Russian anti-communists, but withdrew that assistance when the anti-communists faced defeat.

In November 1919, George signaled the end of British involvement in the Russian Civil War. Britain, George said, could not "afford to continue so costly an intervention in an interminable civil war." That "costly intervention" cost a grand total of 327 British lives from July 1918 to October 1919; the British lost over 900,000 men by some estimates during World War I. That "interminable civil war" resulted in the ascent of the USSR, the germination of World War II, 70 years of terror in Russia and Eastern Europe, and the rise of Communist China, Communist Vietnam and Communist North Korea.

In the British government, only Winston Churchill saw the ramifications of a communist victory. "It is a delusion to suppose that all this year we have been fighting the battles of the anti-Bolshevik Russians," Churchill said in December 1919. "On the contrary, they have been fighting ours, and this truth will become painfully apparent from the moment that they are exterminated and the Bolshevik are supreme over the whole vast territories of the Russian Empire."

The West's failure of imagination in 1919 ended in disaster. Because Lloyd George could not see beyond the horrors of an unpopular Russian war, he became a midwife to the Soviet Union. Because Woodrow Wilson lacked the vision to see the difference between liberalism and communism, he became Soviet Communism's nursemaid.

Failure of imagination remains the greatest danger facing Western civilization. Today, the left's failure of imagination has infected the body politic with regard to the Middle East. The left's myopic obsession with daily body counts has blinded us to basic truths: Iran is an aggressive power bent on domination of its region. Walking away from Iraq creates a power vacuum Iran will quickly exploit.

We can simply walk away from Iraq; we can ignore the burgeoning threat of Iran. But are we willing to bear the consequences? For 20 years after World War I, Britain and America enjoyed the bounty of peace, even as millions perished in Russia. Soon enough, the Soviets joined with the Nazis to threaten Western civilization. After the defeat of the Nazis, the Soviets murdered with impunity for decades more.

On November 8, 2001, President Bush spoke about the events of September 11. "We have endured the shock of watching so many innocent lives ended in acts of unimaginable horror," he said. It was our failure to imagine the possibility of such horror that allowed it to occur. We must not make the same mistake again.