Drow trust no other creatures, including (or especially) other drow. Drow believe all other living beings are inferior. The relations of such a paranoid race with others is uneasy, to say the least. But they don’t just ignore or kill all none drow. They interact with various other Underdark races at least to some extent. Possible equals (the drow admit to no race as their superior) such as illithids and duergar are dealt with by a mixture of armed truces, hard bargaining, subtle threats, magic, and mutual gain pacts. Trade treaties or military alliances might be arranged when the need arises, but they won’t be permanent and only limited to a certain time. Yesterday’s ally might be a good source for new slaves tomorrow. However, they respect the strength of the duergar and illithids, as they are capable of building massive cities compareable to drow ones and were able to withstand more than one drow assault before. They are respected, but still considered inferior.
Only exceptional drow individuals come to trust another being fully (and live to tell the tale!) in the cities of Lolth worshippers. Trusting drow are more common among the worshippers of Eilistraee, but even theirs is hard to gain. The drow are the most widespread and powerful of the known Underdark cultures. Strife among themselves, and a lust for a personal power and commercial gain, always prevents drow from gathering in any concerted effort against foes. This is probably the only thing that keeps many kuo-toa, jermlaine, and disir communities intact; the drow could easily wipe out weaker neighbors if they took up arms in earnest. Oddly enough, rule over all the subterranean races (even to the powerful mind flayers and aboleth) is the ultimate goal of drow - “the First Part of the Destiny of the People,” as priests of Lolth put it. The Second Part is the extermination of all other elven races, including the seizing of surface lands and holdings. Lolth and her clergy do not, however, advocate any sort of “holy war”. Lolth sees her People best served by the competitive striving of individuals, cabals, factions among drow society, her priesthood, and by the efforts of individual drow communities. In this way, the race grows stronger without breeding in decadence, weakness, and laziness.
Their true focus of hate is reserved for the surface elves. Any chance to bring pain and death to them is more than welcome. Other races can expect being captured to serve as slaves - mostly orcs, goblins, hobgoblins, ogres and humans. Almost all basic and unskilled labor in a drow city is performed by slaves. Some prisioners, mainly higher ranking surface elves, might end on one of the many altars.
Drow even practice cannibalism when their numbers grow too great for a given hold or community. Only the aged are used as food, as the survival of the young is seen as the future of the People. Population pressures like this result because sending out expeditions to find new holds is thought too likely to provoke war with powerful neighbors. Plans to further drow rule and influence are supported with the greatest enthusiasm by drow whose present social position is good, if they are not threatened by the plan. Drow of low class are most likely to support plans that involve open warfare, or strife among drow.
If drow are treacherous among themselves, they are even more so in their dealings with others. Drow will readily “negotiate” with other races over matters of boundaries, trade, and peace. They will break any such agreements and treaties the moment that terms no longer seem advantageous (bargains with “lesser races” are not considered binding). Most races have learned to be cautious and alert when dealing with drow, and to have several tricks up one’s sleeve to deal with the treachery when - not if - it comes.
Above all others, deep gnomes hate the drow. Conversely, in all the Underdark, there is no creature the dark elf enjoys slaying more than a svirfneblin. Other traditional drow foes include dwarves, humans, and other elves of all sorts. Duergar, illithids, and lesser races of the Underdark are traded with, but never trusted or befriended, except by the most worldly and experienced drow merchants.
This is not to say that drow are an unruly, unpredictable mob of violent berserkers, engaged in a sort of endless civil war. (Elminster: Actually, a more apt description to some would be that they are a decadent, status obsessed nest of vipers engaged in an endless controlled civil war.) They strive always for personal ascendancy, yes, but their striving is governed by rules and group loyalties.
Even if freed from the authority of an established House, ruler, or realm, drow instinctively band together in groups. Survival, to these warlike folk (so often at sword’s point among themselves) is often a matter of numbers, trusted battle companions, and tactics.