While Israel was staying in Shittim, the men began to indulge in sexual immorality with Moabite women, who invited them to the sacrifices to their gods. The people ate and bowed down before these gods. So Israel joined in worshiping the Baal of Peor. And the LORD's anger burned against them. (Bemidbar / Numbers 25:1-3)
The history of the people of Israel is fundamentally a spiritual one. As a nation created as a result of God's promises to Abraham, Isaac, and Jacob and then rescued by God from oppression in Egypt, Israel was called to be God's people. According to the Torah, the welfare of the nation would be dependent on its faithfulness to God. Israel was also to be the framework through which God would send the Messiah, the Savior of the whole world. Israel's history has also been designed to help the rest of the world understand spiritual principles.
Whenever Israel would turn to other gods, it would inevitably cause them great trouble. As we read in this week's parsha, before entering the Promised Land, Israel was in the vicinity of the nation of Moab. At some point they joined the Moabites in their worship of the false god, Baal, resulting in the death of thousands of Israelites.
Just prior to this incident, we read how Balak, king of Moab, having heard of Israel's earlier exploits, hired a sorcerer by the name of Balaam to curse the Israelites. God intervened and caused Balaam to bless Israel every time he tried to curse them. We may wonder then why God didn't intervene again in order to prevent Israel from turning to Baal as they did. Balaam's attempt to curse them was completely out of their hands. In that case God foiled the King of Moab's evil scheme. But in the case of the Moabite women, there was something else going on.
As we can see from the verse quoted at the start, Israel's sin was not purely religious. It was spiritual, as I will explain shortly, but not purely religious. It was not as if the people were drawn into simply singing songs to Baal, which would be bad enough, but they also engaged in sexual immorality. It is difficult to tell if they were lured into Baal worship by the advances of the Moabite women or if the sexual activity was part and parcel of the worship. Either way, unlike the earlier attempted spiritual attack, the men of Israel willfully engaged in illicit sexual activity.
What isn't obvious from the immediate context is it was Balaam that instigated this sexual activity (See Bemidbar / Number 31:15-17). What he could not accomplish through direct spiritual attack, he did through the advances of the Moabite women.
The lure of illicit sexual activity continues to be a snare to people today. The connection with Baal worship and the resulting destruction should warn us of the seriousness of such activity. Even though secular societies rarely, if ever, acknowledge the spiritual aspect of immoral behavior, that doesn't mean that it is not rooted in evil spiritual forces. The tactics of evil have not changed in all these centuries. Until we accept how serious sexual immorality really is and what its roots really are, the forces of evil will continue to overcome us.