Military action
Carrying on the legacy of their human masters, the machines keep fighting each other relentlessly.
The machines - which were programmed to fight on no matter what - took over the desolate planet. They had become autonomous to the point of being able to produce the things they needed themselves: fuel and other things the machines needed to stay alive, even new AIs. While the factories kept producing new vehicles, the existing ones tried to annihilate each other.