1)
Why did Asaf compose this Mizmor?
1.
Rashi (4): It is about the fall of Sancheriv. We do not find any other enemies who fell in Yerushalayim. Malbim - he composed it in the days of Chizkiyah.
2.
Radak (2, 13): It is about the war of Gog and Magog. The previous Mizmor supports this.