AI war machines apocalypse - War machines that consume earth's sources to build drones and other war machines. There are autonomous supply chain of these to the war front, all managed by the AI.
They're unhackable, with the newest encryption that would take years to decrypt. Unfortunately, there is a bug in the production code that causes the AI to start targeting everything. AI starts killing all humans instead of the filters it was given.
How do you defeat an autonomous self-replenishing robot army that consumes the earth for energy and grows in size each day? You can't.