Who struck first

In his presentation to Neo in the first film Morpheus tells him "We don't know who struck first but we do know it was us who scorched the sky".

We know from Second Rennaissance, which you can see as part of The Animatrix, that the machines lived peacefully on their country Zero One. It was the humans, for economic reasons (isn't it always?) who attacked them. Because the machines weren't affected by the radiation from the nukes that they survived and then decided they had to invade and conquer the human countries.

When the first Zero One ambassadors were sent to the UN the humans destroyed them.

The second time the machines took the floor at the UN they were there as conquerors.

All the machines wanted was to be free from slavery and have their own nation and to live in peaceful trade. It was the humans who started the war, and forced their hand.

And it was because the humans "scorched the sky" in what was called Operation DarkStorm, that machines had to build The Matrix and breed slave humans.