Humans and Truffles met when the Truffles came out of the caves of Earth, and caused mass amounts of death to the humans, who attacked them. Humans eventually learned they are extremely friendly, and only look up to allying with the Humans, despite them killing a lot of Truffles.
Truffles are now allied with the Humans, and frequently trade with them, since they know some ways of making metal alloys that would be extremely difficult, if not impossible to produce with the human's tech.
In turn for the Truffles giving the Humans a metal alloy called "Shroomsteel"(Name subject to change, suggestions needed) they provide them with military support, should they need any. Truffles do not have any military whatsoever.