well intermarriage in judasim is a sin, so.....

to add, the 7 nations were evil, immoral people, and deserved this.

G-d doesnt need a casus belli like us mortals do. he does things for his own reasons. i find it perfectly ok to question human acts, but less ok to question G-d's acts. who are we mortals to question him? he acts in ways we cannot fathom and for reasons we cannot fully explain all the time.