en_tn/isa/62/08.md

by his right hand and by the arm of his strength

This shows God will surely fulfill what he says. AT: "by his power" (See: rc://en/ta/man/translate/figs-metonymy)

Surely I will no longer give your grain as food for your enemies

This means God will not let the enemy conquer the people of Israel and take their grain. Perhaps the enemy took the grain in the past as a tax or to feed their own armies.