In Deuteronomy 28:15–68 the Lord promised that he would send curses upon Israel if they did not obey the terms of the covenant. These curses were meant to make the people realize that they were sinning against God so that there could be repentance. Joshua is reminding the people that just as the Lord kept all his promises to help them in battle and bring them into the land (Joshua 21:43–44), he will also be faithful to the promises he made to bring curses upon them if they should be disobedient.
15 But just as all the good things that the LORD your God promised concerning you have been fulfilled for you, so the LORD will bring upon you all the evil things, until he has destroyed you from off this good land that the LORD your God has given you,