If once I begin to prepare for war, and for the execution of my sentence. Take hold on judgment, i.e. of the instruments of judgment, of the weapons of war. A metaphor from warriors that take their weapons into their hand when they intend to fight.
Related Commentaries of Deuteronomy 32:41
Deuteronomy 32:41
41 If I whet my glittering sword, and mine hand take hold on judgment; I will render vengeance to mine enemies, and will reward them that hate me.