Relations between kings and queens of countries (and therefore between countries) were pretty fluid--they changed often. But some major known enemies of Ferdinand/Isabella's reign were:
France, particularly Charles VIII of France, who invaded Italy and expelled Alfonso II, Ferdinand's first cousin once removed, from the throne of Naples. However, he and the French were on good terms after Isabella's death, which is obvious because his second wife was Germaine of Foix, niece of King Louis XII of France.
"Friends"/allies were:
Italy, particularly Emporer Maximilian I, who helped Ferdinand expel the French from Naples and put Alfonso's son Ferdinand on the throne. Later, though, they went to war with Italy--against Venice (around 1508)
England, at least in 1511, when he and his son-in-law Henry 8th of England signed the Treaty of Westminster, allying themselves together against France--and at least when Ferdinand married his daughter, Catherine, off to Arthur, Prince of Wales (and then on to Henry 8th after Arthur died.) She was eventually put aside by Henry so that he could marry Anne Bolelyn, and that of course angered the Spanish, but I don't think that Ferdinand was alive at the time. During Elizabeth's rule (she was the daughter of Henry 8th and Anne, Henry's second wife), the Spanish were a great threat to England and vice versa
Andreas Paleologus, who left Ferdinand and Isabella as heirs to the Eastern Roman Empire
