Was "Operation Barbarossa" a pre-emptive strike?

On June 21st 1941, Nazi Germany invaded the Soviet Union without a declaration of war, breaking the non-aggression pact of 1939.

The following 4 years saw some of the bloodiest battles in human history, and some of the worst genocide of all time. Over 20 million people lost their lives, but it ended with the destruction of the Nazi regime and their surrender on May 9th 1945.

But is our vision of history true? Was the 1941 invasion merely a defensive act by Nazi Germany against the Soviet regime, which was poised to strike? Some historians posited this theory in the 1980s, and it has been taken up with enthusiasm in some circles, and derided as fanciful fiction in others.

Lets hear arguments both pro and con, and see if we can make up our minds. As the "pro" faction is attempting to overturn the accepted view of history, I will open up the floor to them to make their opening arguments. Lets try and keep it civil and non-personal, cite sources (even Wikipedia: it's a source, while claiming "it is well-known" is not), and keep our arguments short so as not to snowball this.

1,2,3 - Go!