A foe is an enemy. Foes can range from an adversary of one person to that of a nation, from “he is my foe” to “they are my foe.” There is also a “that is my foe” who those who are trying to diet know well, aka chocolate.
A cousin of the word feud (though they don’t get a long of course) foe may refer to a military opponent or a personal nemesis or rival — Lex Luthor is Superman’s foe. Historically, foes often come in pairs — such as the North and the South in the Civil War. The word foe is sometimes used for an opposing concept or a thing — if you are on a diet, chocolate may be your foe.