God had made some wonderful promises to the patriarch Abraham, and God always keeps His word! God gave them the land of Canaan for the time was right (i.e., the sins of the inhabitants of Canaan were overflowing and annihilation was necessary). Israel was victorious not because of their strength in numbers or skill in battle. They won because God wanted them to win! He gave them victory after victory, and they would enjoy continued success ("rest") as long as they stayed true to the covenant.
These verses seem simple enough, but they have significant application even for us today living in the Christian era. Allow me to explain.
There are some today who hold premillennial convictions. In other words, they believe we are currently living prior to the "millennium," and they contend that Jesus will one day return to reign on a physical throne in physical Jerusalem for 1000 years. It is not within our scope at this time to tackle this broad and sometimes varied false doctrine in whole, but rather there is one component of it that Joshua 21:43-45 deals with masterfully.
Those who expect Christ to return for a physical reign understand that He must have a physical land to return to. Thus, many with premillennial leanings support the modern nation of Israel because they believe its existence is a prerequisite to Jesus' return. These same people believe the Promised Land is still owed to the nation of Israel in some way, but the text above says "no."
The promise God made to Abraham approximately 4000 years ago has been fulfilled! It had been fulfilled in Joshua's day! How much plainer can the text be: "The LORD gave to Israel all the land of which He had sworn to give to their fathers...not a word failed of any good thing which the LORD had spoken to the house of Israel. All came to pass"? If God had given everything He intended to give, and the Israelites squandered the land due to immoral living and lost possession of it (as will be shown in the remainder of the Old Testament), there should be no real expectation today that God must restore the land to the descendants of Jacob. Besides, true Israel today is of a spiritual nature, not physical ancestry--but I digress (cf. Rom. 2:28,29).
God gave the land of Canaan to Israel and they would lose it. The land promise was fulfilled, and there is no requirement that it be fulfilled again in the modern era. I wish nothing but the best for any Jew living in Israel today (or anywhere in the world for that matter), but I do not believe the existence of that nation today is required for Christ's return! He's not coming to reign on a physical throne; the faithful shall meet Him in the air (cf. I Thess. 4:16,17)!
There are other verses that underscore the same truth. For instance, Nehemiah 9:7,8 - "You are the LORD God, who chose Abram, and brought him out of Ur of the Chaldeans, and gave him the name Abraham; you found his heart faithful before You, and made a covenant with him to give the land of the Canaanites, the Hittites, the Amorites, the Perizzites, the Jebusites, and the Girgashites--to give it to his descendants. You have performed Your words, for You are righteous."
Finally, Deuteronomy 19 lends additional support to our conclusion that God had completely fulfilled the land promise centuries before Christ lived and that no one today can rightly say that God still must provide the Promised Land for the Jewish people. In that chapter it is explicitly stated that there should be three cities of refuge. However, the nation was told by Moses - "if you keep all these commandments and do them, which I command you today, to love the LORD your God and to walk always in His ways, then you shall add three more cities for yourself besides these three" (Deut. 19:9). So, if six cities of refuge were ever established (and there were six ultimately), then God had fully given the land He had promised to the fathers (cf. 19:8). Almighty God keeps His word!