Albert Barnes Commentary Numbers 25:1

"And Israel abode in Shittim; and the people began to play the harlot with the daughters of Moab:" — Numbers 25:1 (ASV)

The records of the neighboring cities of the plain, and the circumstances of the origin of Moab (Genesis 19:30 and following) suggest that the people among whom Israel was now thrown were more than ordinarily licentious.