This can be stated in active form. Alternate translation: "For when Yahweh speaks he will shatter the soldiers of Assyria" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Isaiah speaks of Assyria's fear as if Assyria is an object that Yahweh's voice shatters. Alternate translation: "Assyria will be terrified" (See: [[rc://en/ta/man/translate/figs-metaphor]])