An unknown catastrophe around this time causes the human race's interstellar confederacy to increase its defensive military posture and begin developing new weapons and military tactics to meet the unknown threat. Many of these technological developments will later be recovered by the forces of the Imperium of Man during the Great Crusade of the late 30th Millennium.
Age of Exploration
During the Great Diaspora of human expansion, Mankind seeks to remake the galaxy in its own image, including seeding worlds with familiar Terran flora and fauna. In many cases, these imported lifeforms eventually replace the new colony worlds' native ecosystems, leaving the human-settled galaxy with an unusual level of biological homogeneity.
During the Dark Age of Technology the first Titans are created. These massive, crewed, robotic bipedal combat walkers. These immense war machines can stand hundreds of metres tall, and often instilled terror by their mere presence. There was little that could equal their armament and sophisticated technology. When Mankind continued further expanding into the unknown reaches of the galaxy, they encountered several xenos races that were often times deadly or extremely hostile. At that time each human colony had at least one Titan Legion, and many had dozens deployed in their defence. Titans played an instrumental role in protecting these early colonies from the depredations of these dangerous xenos, playing a pivotal role during the first Alien Wars.
When mankind reached out to the stars and settled many far away worlds, among the apparatus of colonisation were formidable war machines known as Knights, which existed to protect the settlers from any threat. These small combat walker are one-man versions of a Titan.
The so-called "Iron Men" or "Men of Iron," artificially intelligent humanoid machines, are created by Mankind at this time. The Men of Iron were loyal only to Mankind, and served as humanity's army in the period when much of human space was united by a federation-type government that existed before the Imperium of Man. The Men of Iron were developed after the similar artificially intelligent constructs remembered only as the Men of Stone, but before the "modern" conception of robots. By the 23rd Millennium, extant records cite betrayal by the machines and a great war with robotic armies.