Or HaChaim on Deuteronomy 20:4/אור החיים על דברים כ׳:ד׳

1 להלחם לכם..להושיע אתכם, "to do battle for you with your enemies and to save you." The meaning of להלחם is to destroy your enemies, whereas the additional להושיע אתכם means to ensure that not one of you becomes a casualty of war. This last point is always the essence of the miracle by means of which we know that G'd fights on our side. This may be the reason that at the beginning of the verse the Torah mentions ה׳ אלוקיכם emphasising that both the attribute of Mercy and the attribute of Justice are involved here. The attribute of Mercy expresses itself by saving the Israelites, the attribute of Justice by destroying its adversaries. Had it not been for the Torah wanting to emphasise this it only had to write כי אלוקיכם הולך.