What is "va'Tazreni Chayil la'Milchamah"?
Radak: (You girded me with strength for war.) It should say va'Te'azreni (with an Aleph), like "va'Te'azreni Simchah" (Tehilim 30:12). For ease of pronunciation, the Aleph was omitted, and its Nekudah (Patach) was moved to the Tav. 1
Radak citing his father: This is Hifil, from the root Zere. You cast strength into all my limbs to make war.
Malbim (Tehilim 18:40): In the first method, He girds him with strength, but it is not used for war (he does not fight). By contrast, in this method the strength is used for war.
And so it says in Tehilim 18:40.
Why did he say "Tachri'a Kamai Tachteni"?
Malbim (Tehilim 18:40): Even though in this method the strength is used for war, You lower those who rise against me "Tachteni", i.e. You do so in place of me.
