I don't see why their motivations would be any different then why regular humans go into the mercenary business.
Or are you saying the entire race are serving as mercenaries for Fantasy Britain?
Needs To Do Her Work!
Pretty much, most got jobs with the Empire, but some went to work with other governments, and some went into hiding. I guess mercs is too strong of a word, agents would be more appropriate.
But Don't Forget Knuckles O'Shaughnessy!
Well, considering you described that some have been driven into hiding in order not to work for Fantasy Britain (which would suggest an element of coercion) and there sounds to be a dichotomy between the ones that work for Fantasy Britain and the ones that don't, I think it might be a tad difficult not to paint Fantasy Britain as the bad guy.
As for "We need help, so we'll help you", who is the one offering help and who is the one receiving help in that statement? If your magical race is the one in need of help, think of a want or need that they would have that only this steampunk empire can supply.
Ultimately, I think the best person to come up with a reason is yourself since no one else knows enough about either the empire of the fantasy race to know their motivations, priorities, needs, wants, capabilities, histories, etc. Ideally, you should try to find a reason that is relevant to your story.
But in terms of generic ideas, perhaps something along the lines of technology (which would be the steampunk empire's forte), medicines (e.g. for some new disease ravaging the magical race) or land (were they ousted by a third force or in need of refuge?). Otherwise, there's always glory, god and gold (i.e. Glory: prestige if they are a proud, warrior race; God: a common religion that units them and/or; Gold: money, with greed motivating the offer of assistance).
Also, look up Gurkhas
for inspiration. Bonus points as they are closely tied to Real Life Britain and its empire before that.
edited 16th Aug '12 3:49:07 AM by peasant
In this Steampunk story I'm writing, long ago (So this part would be greatly pre-Steampunk) there was this race called the Oibri-Beocht who have more strength and agility than regular humans, and have the ability to harness this magic energy to power stone weapons and stuff. Something needs to happen so the OB would come into an agreement with a human Empire (Who are a stand-in for Britain pretty much), and would act as there mercenaries/spies/ninjas. What would drive the OB to make such a deal?
Notes:
1. I don't want to make the Empire bad guys (So no "they conquered the magical people"). It'd be more of like "We need help, so we'll help you" type of deal. 2. The Oibri-Beocht have a nature loving, animistic type of religion, so factor in that of you need to.
But Don't Forget Knuckles O'Shaughnessy!