Last week's parsha warns us that if we don't drive the Canaanim out of Eretz Yisrael, "V'haya asher tosiru meihem l'sichim b'eineichem u'l'tzeninim b'tzideichem," they will be like splinters in our eyes and thorns in our side.
Chazal tell us as a general rule that when a sentence or parsha starts with the word "v'haya" it portends good tidings -- "ain v'haya elah lashon simcha." What's the simcha in our pasuk? What kind of simcha is it to be poked in your eye or side?
R' Tzadok haKohen (Pri Tzadik, Matos #5) quotes the Pesikta on the haftarah which explains that when Bnei Yisrael entered Eretz Yisrael, they left alive Rachav, who in turn (through her family) produced seven prophets, among them Yirmiyahu haNavi. It is the words of Yirmiyahu haNavi that are the thorn in our side! It is his tochacha and mussar that are painful for us to hear. Bnei Yisrael were so upset by Yirmiyahu's words that they threw him into a pit to get rid of him.
At the same time, "v'haya," there is great simcha in shamayim from those words, because it is through those words of tochacha that we will eventually get out of galus. Had they penetrated back when they were delivered, maybe there would not have been a galus, but even if we didn't listen then, we've had 2000 years to hear them again and again, and eventually they will sink in.