In Charles Stross's Laundry Files there is a peace treaty between each territorial government with a coast, and the entities covered by the codename BLUE HADES.
This comes up in "The Jennifer Morgue" and in is mentioned again in "The Rhesus Chart".
Said entities, covered by codename BLUE HADES, are almost definitely H.P. Lovecraft's Deep Ones. more or less. Bob sums up the treaty thusly:
We have an agreement with them: we agree not to piss them off (by, for example, building the Underwater City of Tomorrow! or otherwise intruding on their territory without prior arrangement) and they agree not to exterminate us
What is there reason for the treaty?
Is it that they are nice and don't want to kill people?
Is humanity a threat to them that is better dealt with peacefully? Perhaps because what few humans they couldn't kill off, (non-coastal) would retaliate to an extent that BLUE HADES would take greater losses?