"V'nashal Hasem Elokecha ess hagoyim ha'el m'at m'at, lo suchal kalosam maheir pen tirbeh alechah chayas hasadeh. (7:22) - And Hashem will wipe out the goyim from before you slowly but surely, but you cannot destroy them quickly, lest the animals of the field increase upon thee".
I always wondered about this. After all, if Hashem can bring the 10 plagues on Egypt and split the sea, why can't he wipe out the goyim in Eretz Yisrael in one fell swoop? Is Hashem really worried about the increase of the animals? Is that something harder to deal with then getting rid of these great nations?
It occurred to me that maybe there is a far deeper meaning here. All the creations of the world have a connection to Hashem based on their needs. The more someone "needs" Hashem, the more connection they have with him. Hashem told the snake after the sin of the Etz Hada'as "and you shall eat the dust of the earth all the days of your life", and Rashi explains that Hashem wanted no connection with the snake and therefore gave him his food wherever he goes. However, the other animals of the field need to ask Hashem for food every day, as it says in Tehillim "Livakesh Mikel Ochlam" - "they ask from Hashem their food".
Now Hashem wanted human beings to have even more connection with him than the animals, and therefore humans don't have a natural way to get food like the animals do, but rather are dependant on owning land, toiling the soil, rainfall, and on a good crop and harvest. In last week's Parsha, one of the praises mentioned of Eretz Yisrael is "Limtar Hashamayim Tishteh Mayim" - "From the heavens you will drink water", and the Pasuk goes on to say - "not like Eretz Mitzrayim which drinks like a watered garden from the Nile". But why is that a praise? It would seem that Mitzrayim is more fortunate than Israel! The answer is that Hashem wants more of a connection with us than he wants with the Egyptiams. To them he gave them the Nile river so that they don't need rain fall at all and don't need Hashem at all. However, Eretz Yisrael drinks from the heavens, and like it says; "the eyes of Hashem are on the land [of Israel] from the beginning of the year until the end". Eretz Yisrael needs special divine intervention for water, and the praise of this is that the Jewish people living there are always dependant on Hashem for rainfall and therefore remain strongly connected with him. After all, like it says in last weeks Parsha, "and you will receive buildings that you didn't build, wells which you didn't dig, vineyards that you didn't plant, etc..." and the Yidden will have everything they need in Eretz Yisrael. If they weren't dependant on Hashem at least for rainfall, they would no longer need him and quickly forget him.
The same applies with our enemies. And that is why it says in this weeks Parsha that although Hashem will wipe out the goyim from before us slowly but surely, still, he will not destroy them fast - "lest the animals of the field increase upon thee". What this perhaps means is, that if Hashem would wipe out our enemies all at once, we would no longer feel a need to depend on him. The words "lest the animals of the field increase upon thee" mean to hint perhaps, that if Hashem would destroy our enemies fast then even the animals of the field would "increase" over us, meaning that the animals would have even more of a connection with Hashem than we would.
And that is why Hashem has given us the Yetzer Hara as well. He wants a connection with us! He wants us to know that we need him, and that without his constant help, we are lost. And that is also why Hashem doesn't destroy the Yetzer Hara in one fell swoop once a person decides to do Teshuvah. Instead, each time we think we got rid of him, he keeps coming back again and again. Only "slowly but surely" does Hashem wipe him out from before us. For if Hashem would get rid of the Yetzer Hara all at once, we wouldn't need him any more and we wouldn't feel dependant on him. And this "connection" that we have to Hashem through our struggles with the Yetzer Hara, is even more important to Hashem than the falls that we have as a result of Him not removing the Yetzer Hara altogether as soon as we want to do Teshuvah.
To sum it all up: The most important thing to Hashem is not our progress in destroying the Yetzer Hara, but rather our dependency on him, and our constant knowledge that we need Hashem every day anew to help us break free of our #1 enemy.