In Numbers 25 we are given the account of the children of Israel falling into sexual sin with the women of Moab ( verse 1) and evidently of Midian too ( verse 6 and Numbers 31:16). That led to idolatry as the foreign women invited the children of Israel to the sacrifices of their pagan gods and the children of Israel bowed down to them ( verse 2 ).
One sin led to another, it so often does. When we let our guard down in one area, it so often opens the floodgates that soon has us far deeper in sin than we ever intended to go.
This all happened as a result of Balaam, who having been unsuccessful in cursing the children of Israel in the previous 3 chapters on behalf of Balak king of Moab, advised Balak to introduce Moabite women in amongst them to seduce them morally which would then lead them into idolatry ( Numbers 31:16 & Revelation 2: 14).
The plan worked. Balak could not bring harm on the children of Israel through Balaam’s curses, so he got the children of Israel to bring harm on themselves through sin, which led to God’s judgment in the form of a plague, that killed 24,000 people ( verse 9).
If our enemy Satan can’t bring harm on us himself, he will try to get us to bring it on ourselves, by luring us into sin and the spiritual vulnerability and harm it leads to.
If he can’t attack us from “without”, he attacks us from “within”, via our own hearts, tempting us to sin, with the death and destruction it leads to if we fall.