God called Abraham out of his home in Ur to inherit the land of 7 nations called Canaan (Gen 12:1-3 & 7, Gen 13:14-17, Gen 15:13-16, Gen 17:8 & 19-21, Gen 21:9-13). This promise was passed down to his descendants Isaac and Jacob (Gen 26:1-5, Gen 28:4 & 13-15). God told Abraham his descendants would first migrate to another land where they would be mistreated for 400 years, and they would then return to receive this promise (Gen 15:13-16). This turned out to be Egypt (Gen 37-Ex 1).
When the 400 years was over, Go raised up Moses to lead the Israelites out of Egypt into the promised land (Ex 2-4). Moses warned the Israelites several times that the inhabitants of the land were very wicked, and would have to be destroyed and driven out, lest they learned their sinful ways and forsook their God (Ex 23:20-22, Ex 34:11-16, Num 33:50-56).
Initially, the Israelites saw great conquest upon their entry into Canaan as Joshua led them in battle (Josh 1-12). However, when Joshua was old, the Israelites showed slackness in finishing the job. 7 tribes had to be pushed to go inherit their portions of the land, and they left many of the people which were to be driven out and destroyed in the land (Josh 13-Judges 1). The Israelites did exactly what Moses warned them against, and learned the ways of the pagan, sinful nations they left in the land (Judges 1-2). As a result, the Israelites went on a spiritual roller coaster ride in a cycle of sin, repentance, deliverance, and backsliding. One of the nations they were to have destroyed but did not was the Philistines (Ex 23:31, Josh 13:2-3, Jud 3:1-9). We see the Philistines continually being an enemy to Israel throughout the rest of the Old Testament (Jud 3:31, Jud 10:6-11, Jud 13-16, 1-2Sam). The prophets toward the end of the Old Testament still mention them (2Ki 18:8, Ezra 9:1, Jer 47, Eze 16, Eze 25:15-16, Amos 1:8, Zeph 2:5, Zech 9:6). We read of David's great battles against them including Goliath, but we forget that he should have never had to fight those battles. The Philistines should have been long gone if the first generations of Israel in the promised land had done their job.
The promised land presents a picture of us in the New Testament (Heb 3-4). The new covenant does not promise Christians a geographical territory in this world, but a spiritual one. The promise is the gift of the Holy Ghost (Lk 24:29, Acts 1:8, Acts 2:1-4, Acts 2:17 & 33 & 38-39, Gal 3:14, Eph 1:13). God gives us victory over the power and effects of sin, but when we receive the Holy Ghost, we are not promised there will be no battles. In fact, God purposely makes sure we have some battles in the promised land (Deut 7:22, Judges 3:1-4). This is for our benefit in at least two ways. It teaches us that we need to fight for what God has promised us, and it tests us to see if we are going to keep God's commandments or not; for "the kingdom of God suffereth violence, and the violent take it by force (Mt 11:12)." Some things we used to fall prey to may never trouble us again after we are saved. We may have to battle other things for years. God does not set us up for failure, but victory (Rom 8:35-38, 2Cor 2:14). However, God is not going to allow us to just float along with no battles while He does everything for us. We are laborers together with Him (Mt 9:37-28, Mt 20:1-8, 1Cor 3:9). We must be aggressive in driving out those things that do not belong in our character or behavior. They will not go voluntarily. We can grow comfortable with those things which we allow to remain until we accept their presence. They will eventually corrupt us. A little yeast will spread through the whole loaf if not removed (Mt 13:33, 1Cor 5:6-8, Gal 5:9). There is no such thing as a little sin or a little false doctrine. As with Israel in the promised land, this is a process. So we should not be discouraged by the time it may take to drive things out. We should focus on doing our diligence, and not slacking off. God will renew our strength if we keep making the effort (Is 40:31).