Why does it say that Hashem will give to him Ru'ach?
Malbim: (a) He will believe what he hears, even though it was not a substantiated report. (b) He will return with his mammoth army to his land, and not merely send some of them.
What will he hear?
Rashi: He will hear that Tirhakah Melech Kush came to fight him.
Will he return to his land due to what he hears?
Rashi: No. Due to what he hears, he will leave Yerushalayim and go to Kush. Later he will be defeated in Yerushalayim, and return to his land 1 in shame.
Radak (Melachim II, 19:7), Malbim: Yes. Melech Kush went to Ashur to fight there.
It says "ba'Derech Asher Ba Bah Yashuv" (verse 34). Rashi must explain, the same way he came to Yerushalayim the first time, he will return to Ashur after his defeat. However, how can he explain "ba'Derech Asher Yavo Bah Yashuv" (Melachim II, 19:33)? He will come (return to Yerushalayim) from Kush, and return to Ashur! Perhaps Yavo really means past tense (came). This requires investigation. (PF)
Why didn't Hashem wipe out Sancheriv now, rather, only after he defeated Melech Kush and returned to Yerushalayim?
Rashi (Melachim II, 19:7, from Seder Olam 23): It was so Yisrael would get the booty that Sancheriv took from Melech Kush and Melech Mitzrayim after he defeated them - "Yegi'a Mitzrayim u'Schar Kush u'Sva'im Anshei Midah Alayich Ya'avoru u'Lecha Yihyeh" (Yeshayah 45:14).
Radak (Melachim II, 19:7): We cannot question Hashem, who has awesome reasons. It seems that it was (a) so Yisrael would get the booty from Kush and Egypt 1 , and (b) so Kush and Egypt would see the wonders that Hashem does for those who love Him and seek His name. Melech Ashur defeated them and the other nations 2 , and in Yerushalayim he was stricken without a sword or spear!
Malbim: There is a time for everything. It was not yet Sancheriv's time to fall.
Radak explained that Sancheriv returned to Ashur to fight them. Why did he take the booty to Eretz Yisrael? Rashi (Melachim II, 19:7) explained that Sancheriv fought them in Kush. He returned to Yerushalayim before reaching Ashur to leave the booty there (e.g. if Kush is Ethiopia and Ashur is in Asia, e.g. Syria. - PF)
Even had he not defeated Kush and Egypt, people would see that Melech Ashur defeated other nations, and fell to Yerushalayim without weapons! Perhaps Kush and Egypt were far greater than the other nations that he defeated. (PF)
