Sometime in high school or junior high, I was assigned a short story
to read which was set in some desert (Spain comes to mind) in which
the protagonist met with an acquaintance (not necessarily a friend;
perhaps a rival) before setting off on a trip across the desert on
horseback. Part way through the journey, he went to take a drink of
water from his canteen and found that the acquaintance had filled it
with wine, instead of water, thus ensuring that the protagonist would
die of thirst in the desert. I'd like to read this story again and
have searched and searched, but can't find it. At first I was thinking
Edgar Allan Poe or Jack London and looked through their anthologies,
but they don't seem right.

I am certain that the story you are remembering is "Wine on the
Desert," by Max Brand, which is often included in textbook anthologies
of short stories.

I have not been able to find this story online, but I recognize it
from the details you've given.

Here's a description that should help to verify that this is the
correct story:

"Wine on the Desert 
Setting:  American Southwest, early 1900?s 
Main Characters: 
Durante - Fugitive from the law, supposed friend of Tony,
Tony - Owner of the vineyard, has a wooden leg, expert marksman,
friend to Durante

Plot: 

Durante arrives at Tony's to elude the posse.  The sense of dryness
and the overwhelming nature of the desert from the beginning.
Tony's wife is visiting friends, Durante makes a strange comment about
her being gone
Tony offers to feed Durante rabbit, which he cleanly shoots in the
head
Tony tells Durante the story of how his father died of thirst
(Foreshadowing)
Right after he tells of how his father died of thirst, he offers
Durante wine (Foreshadowing)
The next day they have breakfast, Durante takes Tony's gun, shoots the
barrels, gets the canteen from Tony and leaves
Durante throws the rifle down about a mile from the vineyard,  looks
back and sees that Tony has picked it up
He then takes a sip from the canteen and realizes that the water is
wine
Durante panics, makes the horse gallop itself to death, and runs off
into the desert.
Durante dies the same way as Tony's father. It rains shortly  after he
dies."

St. Christopher's Intranet (cached)
http://216.239.51.100/search?q=cache:NRKFAWBflrQC:www.stcva.org/Intranet/Faculty/MiddleSchool/BaylorC/StoryNotes/WineontheDesert.doc

If you'd like to reread the story, you might want to buy this
inexpensive paperback anthology, which contains "Wine on the Desert"
and 20 other classic short stories:

Amazon.com
http://www.amazon.com/exec/obidos/tg/detail/-/0451627857/qid=1048270652/sr=8-1/ref=sr_8_1/002-6250548-9587205?v=glance&s=books&n=507846

Search terms used:

"wine on the desert"
"max brand"

I hope this is the story you're thinking of. If it is not, please
request clarification before rating my answer, and I will be glad to
resume the search.

Best wishes,
pinkfreud

Thank you for your very thorough answer! It's right on target. And
fast, too. I would never have found it myself. Much appreciated.