system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] When did humanity first come in contact with xenos?

When did humanity make first contact with xenos? What were humanities interactions with xenos like prior to the great crusade?
gpt
The first known contact between Humanity and Aliens as sometime in M18, after the development of the first Warp Drives allowed humans to rapidly expand outside of it's local stellar group. 

This led to the first 'Alien Wars', but also to the development of treaties and alliances with various Xenos species throughout the galaxy. Because Humanity was not united into a single political entity, individual federations dealt with various aliens as they pleased, with Orks being a notable exception, as they were almost always hostile. 

During the Age of Strife, some alien species took to preying on the isolated human worlds, which is what drove the Great Crusade and later Imperial policy regarding Xenos. It wasn't all species, of course, and some worlds maintained very close relationships with aliens during the period, until being destroyed by the burgeoning Imperium.