Major Fletcher combating a large machine
The Machines origins at this point in time are unknown. However many of them are bipedal and it's rumored that their attempts to reach a new revolutionary state of AI is what is encouraging them to utilize the human controlled Sanctuary for their own advancement. They're capable of charging their opponents as well as using firearms to bring down their enemies. Some are shown to be much larger than humans, while others are no bigger than small to medium sized dogs.