What exactly are the conditions for losing ?
In particular, how many nations need to be below the minimum happiness ?
I finally understood what those percentages mean in the difficulty description. A nation gives you happiness * its max funding credits at the start of a month, so that nation's exact happiness can be calculated. Should be explained better. I happened to get that warning about a nation being below the happiness minimum. However, the game didn't end right away at least, and the nation is now above again. Until radar coverage is sufficient, it's problematic to take care for *all* nations.
That happiness minimum is kinda useless btw, since you desperately need the money anyway, so nations should all be exuberant.
EDIT
I found it in the source. Seems that with 8 nations, 3 are allowed to be below the happiness limit.
There seems to be a bug btw, description for NATIONBELOWLIMITPERCENTAGE implies 4 are allowed, but >= is used, thus it ends already when nationBelowLimit is 4, see cp_campaign.cpp line 395 and following. Most of the time at least...unless that defined constant creates rounding error etc.