Joshua 23:15
Therefore it shall come to pass, that as all good things are come upon you, which the LORD your God promised you; so shall the LORD bring upon you all evil things, until he have destroyed you from off this good land which the LORD your God hath given you.
This verse belongs to Joshua's final address to Israel, spoken long after the Lord had given the people rest from their enemies around them. It was given when Joshua called together Israel's elders and leaders to hear his last words.
Just as every good thing God promised Israel had come to pass, so He would just as faithfully bring the threatened evils upon them if they broke His covenant and departed from Him. God's faithfulness applies equally to His promises and to His warnings. Their transgression would kindle the Lord's anger against them. Their disobedience would bring calamity upon them and bring desolation and drive them from the land. This would lead to their swift destruction and removal from off the good land which the Lord their God had given them.
The warning guards against idolatry and against intermarriage with idolaters, and even against approaching sin. It shows that turning from God leads to ruin. It is to be taken personally.