Obama is historically illiterate when it comes to history

Robert Spencer:

5 Ways Muslims Have Contributed to 'Building the Very Fabric of Our Nation'

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This one predates the United States as a nation, but without it, the United States would not exist. Every schoolchild knows, or used to know, that in 1492 Christopher Columbus sailed the ocean blue and discovered America while searching for a new, westward sea route to Asia. But why was he searching for a new route to Asia? Because the fall of Constantinople to the Muslims in 1453 closed the trade routes to the East. This was devastating for European tradesmen, who had until then traveled to Asia for spices and other goods by land. Columbus’s voyage was trying to ease the plight of these merchants by bypassing the Muslims altogether and making it possible for Europeans to reach India by sea.

So the bellicosity and intransigence of Islam ultimately opened the Americas for Europe – and made the United States possible.
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Muslim slave traders were also largely responsible for supplying slaves in the Americas.  Obama has often talked like he either skipped history courses or just makes it up.  This is another example.