This verse is a good example to point out the importance of reading a passage completely in the context in which it was written. That means we have to look at the verses or maybe even the chapters around it to fully understand what it is saying. Joshua 23:15 reads "Since every good thing the Lord your God promised you has come about, so He will bring on you every bad thing until He has annihilated you from this good land the Lord your God has given you." This verse by itself does leave us scratching our head. In one breath Joshua says the Lord has blessed the people of Israel and immediately he follows it by saying God will annihilate them from the land. So which is it? Is God going to bless Israel or curse them? The answer is simple, both.
Thankfully in this instance of studying scripture the context of the passage provides our answer. Lets look at verses 14-16 as a whole 14 "I am now going the way of all the earth, and you know with all your heart and all your soul that none of the good promises the Lord your God made to you has failed. Everything was fulfilled for you; not one promise has failed. 15 Since every good thing the Lord your God promised you has come about, so He will bring on you every bad thing until He has annihilated you from this good land the Lord your God has given you. 16 If you break the covenant of the Lord your God, which He commanded you, and go and worship other gods, and bow down to them, the Lord’s anger will burn against you, and you will quickly disappear from this good land He has given you.” Joshua is giving this address to the people of Israel near the end of his life and he is reminding them how good God has been by keeping his promises to them (Exodus 6:6-8). Verse 15 is a bridge between verses 14 and 16. The beginning of verse 15 speaks of the greatness of God's fulfillment of His promises in light of what was just said in verse 14. The second half of verse 15 speaks of the curse that will come on Israel if they are disobedient to the Lord. This is leading us to verse 16 where Joshua goes into more detail on what will happen if they break their covenant with God. In particular if they abandon God and worship false gods. Sadly God's people did just that as we see in the book of Judges which follows Joshua and again later on when the Assyrians overtake the northern kingdom of Israel (2 Kings 17) and when the Babylonians overtake the southern kingdom (2 Kings 24-25).
I know I have been guilty of getting stuck on a verse trying to figure out what it means only to find that when I finally give up and read on to the next verse or two the explanation is right there in front of me. This verse would fall into that category if we stopped at verse 15. Verse 15 may seem like it is contradicting itself, but when viewed in the context of verses 14 and 16 we see that it is speaking of two separate scenarios, continued blessing on Israel if they are obedient and curses if they are not.