1)
Did they never destroy the nations?
1.
Rashi: They did not do so in the days of Yehoshua.
2)
What had Hashem said about those nations?
1.
Rashi: "Lo Sechayeh Kol Neshamah" (Devarim 20:16), "Lo Yeshvu b'Artzecha" (Shemos 23:33). Yisrael let them dwell among them, to pay taxes.
2.
Radak: "V'Im Lo Sorishu Es Yoshevei ha'Aretz Mipneichem v'Hayah Asher Tosiru Mehem [will be thorns in your eyes]" (Bamidbar 33:55).
