Why Do the Machines (in The Matrix) Have to Maintain the War With Humans?

I've never been clear on this, even after seeing all three Matrix films.

I understand that there's been a cycle where some humans don't do well and don't actually accept the Matrix and break out and start living in and building Zion. I know this cycle has been repeated, so what there is of Zion wasn't just built by the rebels in the current cycle. This also means Neo is only "The One" for this generation, that there's been previous versions of "The One," and after each rebellion, the cycle starts over again.

Why is this cycle necessary? When given a choice, do so few humans opt to stay in the Matrix that the Machines can't survive on the power they supply? Is there a reason the Machines and humans can't create an equilibrium, where those that want to live in Zion can live there, while all other humans stay in the Matrix?

Is there a reason the humans and machines have not agreed to a long term truce that allows free humans and Machines to live without interference and allows humans that prefer the Matrix to live in it and power the Machines?

Answer

It's not a matter of necessity, it is a matter of caution. In similar fashion to BSG, the war is driven by machines that do not benefit from the passage of time and its erosion of human memory. If you watch the Animatrix it shows that the foundation of the war was the humans' inability to see robots as a valid form of life, and the humans' attempt to eradicate the now sentient machines.

However unlike humans and various exterminations of our past, these events did not ocurr to some distant ancestors of the current machines, it happened to THEM. In their eyes humanity had a chance for equality that failed, the only other option was to eradicate.

Then humanity scorched the sky.

Now they had a two fold problem, how to deal with the humans, whom can not be trusted, and how to continue to survive on limited power. Thus we arrive at the solution they have. You can not convince them human kind has "learned". We are the same race that tried to wipe them off the earth, twice. We are the same race that destroyed the life giving powers of our own planet, assuring our own eventual destruction, simply because we could not stand to let the machines have power, their one city.

The Architect is not burdened by human emotion, he does not see forgiveness or redemptive value in humans, all he sees is problems, and solutions. And those problems are absolute, and sustaining.

Personally, I think they should have exterminated the entire human race and just used cows. But then again, I am a Cylon (spoilers).