According to the book of Genesis, the Moabites are descendants of Lot, Abraham’s nephew and sometime acolyte. While they appear throughout the Hebrew Bible as enemies of the Israelites, a Moabite, Ruth, is also the ancestor of King David. Bruce Routledge gives some historical background to the ambiguous relationship between the two peoples:
Moab was a land, a people, and a kingdom located east of the Dead Sea in what is now the kingdom of Jordan. Moab as a land is first mentioned in the reign of the Egyptian pharaoh Ramses II (circa 1270 BCE). The kingdom of Moab emerged in the 9th century BCE and disappeared a few decades after the destruction of Jerusalem by the Babylonian king Nebuchadnezzar II in 586 BCE. . . .
One of the most important inscriptions related to [the history of ancient] Israel derives from the Moabites: it is the Mesha stele, discovered at the site of Dhiban (in present-day Jordan) in 1868. This inscription commemorates events in the reign of King Mesha of Moab, including his overthrow of Israelite rule (see the parallel account in 2 Kings 3). The language of this inscription is very close to biblical Hebrew; in fact, Israelites and Moabites probably could have conversed without a translator.
The Mesha stele also reveals a theological view of history similar to parts of the Hebrew Bible. History unfolds . . . on the basis of the chief god’s anger or pleasure. [According to the stele], Moab was oppressed by Israel because the main Moabite god, Kemosh, was angry with his land, whereas Mesha’s success was the result of Kemosh’s favor.
Read more on Bible Odyssey: http://www.bibleodyssey.org/places/main-articles/moab.aspx