Numbers 25:1

In Shittim. This was its name.

Acting promiscuously with the daughters of Moav. As a result of Bil’am’s advice, as stated in “Chelek.”

And Israel dwelt in the place which is called Shittim, on account of the (Shetutha) foolishness and depravity which were among them. And the people began to profane their holiness, and to strip their bodies to the image of Peor, and commit fornication with the daughters of the Moabites, who brought out the image of Peor, concealed under their bundles.

AND ISRAEL ABODE IN SHITTIM, AND THE PEOPLE BEGAN TO COMMIT HARLOTRY WITH THE DAUGHTERS OF MOAB — “because of the advice of Balaam, as is related in [the Chapter of] Cheilek.”, meaning: “I will counsel thee how to lead the people astray, and I will tell you what this people shall do …” Ramban explains the phrase “I will counsel thee” differently. This is Rashi’s language. And indeed this [seduction to] immorality was not instigated as a plan of the [Moabite] women, but was done upon the advice of their men and their leaders. [The idea] came to them from the elders of Midian, as it says of the Midianites, for they harass you, by their wiles wherewith they have beguiled you in the matter of Peor, for they beguiled them by means of immorality in order to draw them astray [to worship] Baal-peor. Therefore it is [indeed] possible that Balaam was behind this counsel, since he was considered by them [the Moabites] great in counsel, and his intention was to bring evil upon Israel, and therefore he did everything in his power to this end, as it is said, And the Eternal thy G-d would not hearken unto Balaam, and therefore they slew him with the sword.
But according to the simple meaning of Scripture, it is not alluding here to the counsel of Balaam, but only [alludes to it] when it says afterwards, Behold, these [women] caused the children of Israel, through the counsel of Balaam etc.,, Verse 16. just as it does not mention here the counsel of the elders of Midian, and only [mentions it] when it says, for they harass you. Thus [it is only] after the event [had happened], at the time of [describing] their punishment, that He mentions what was the source of the evil that befell them. He is [thus] saying that what happened [to them] as a result of the desire for sexual pleasure which exists naturally in men and women from [the time of] their youth, was only [the result] of an evil plan [whose purpose was] to lead them astray [into worshipping Baal-peor]; therefore [the instigators of this plan] deserve a severe punishment. The interpretation of the verse, Come and I will counsel thee what this people shall do to thy people in the end of days is thus indeed as I have explained [there].
It is also possible according to the simple meaning of Scripture that Balak at first wanted to curse them [the Israelites] and to wage war against them, and he did not want to give them permission to enter his borders at all. But when Balaam told him that he would not prevail over them, and informed him that they would only destroy his land and his people in the end of days, then he [Balak] brought forth bread and wine in the plains of Moab [i.e., in Shittim], and enticed them [with the daughters of Moab] as if he were their friend. This is [the meaning of the phrase] ‘bidvar’ (through ‘the word’ of) Balaam,, Verse 16. for it was because of his words [i.e., his prophecy that the Israelites would not conquer their land now, that the Moabites] did so [i.e., that they did not fight them, but tried to seduce them and lead them astray through their women]. But because it was Balaam’s desire to curse them, and he allowed Balak to hire him [and would indeed have cursed them] were it not for the righteous acts of the Eternal Who turned the curse into a blessing, therefore they slew him with the sword, for both the hirer [Moab] and the hired one [Balaam] were punished, just as He said, and because he hired against thee Balaam the son of Beor etc.

ויחל העם לזנות, originally, there had been no intention of committing idolatrous acts at all. All that the males had intended was to indulge their libido with the womenfolk who made themselves available. However, these people fell victim to precisely the warning of the Torah in Exodus 34,15-16 of what would happen if Jews would allow the Canaanites to remain in their country and conclude a covenant with them. They would be invited to their social gatherings resulting in their eating forbidden foods, and eventually intermarriage followed by lip service to their gods.

ויאכל העם וישתחוו לאלוהיהן. This was a classic demonstration of how the evil urge works, first suggesting minor infractions of Torah law and then, gradually, suggesting major sins. (compare Shabbat 108)

וישב ישראל בשטים ויחל העם לזנות, “Israel had settled down at a place known as Shittim, when the common people profaned themselves by whoring.” It is well known that whenever the Torah commences a paragraph with the word: וישב, what follows is some kind of disaster. Compare Genesis 37,1 when Yaakov “settled” down and the disaster with Joseph followed and he was sold by his brothers. When in chapter 50,22 of Genesis Joseph is described as having settled down, this is followed by his announcing to his brothers his premature death. (Genesis 50,24) When Israel is described as having settled down in Egypt, (Genesis 47,27) this is followed shortly by the report about Yaakov’s (premature) sickness and death. (verse 29) In Kings I,5,5 we read that the people of Israel including the tribe of Yehudah had settled securely, this is followed by a report in chapter 11,14, by: G–d arranging a revolt by the King of Edom against Solomon’s kingdom. [The Edomites had paid annual tribute to the state of Israel since David’s time. G–d had arranged this as His response to Solomon allowing his wife to erect an altar to the Moabite idol chemosh. Ed.]

בשטים, according to Rabbi Elazar this had been the name of that place already previously. Rabbi Yoshua disagrees and says that it was so named after what happened there, i.e. that many Israelites committed the ultimate folly, shtuss, of sleeping around with gentiles and prostrating themselves to idols; [in order to please their partners who considered this as payment for granting them sexual favours. Ed.]

ויחל העם לזנות. They profaned themselves by falling victim to the advice Bileam had given to Balak to lure them into harlotry, a great sin the eyes of the Jewish G–d. (Compare Talmud tractate Gittin, folio 56)