Or HaChaim on Leviticus 26:20/אור החיים על ויקרא כ״ו:כ׳

1 ותם לריק כחכם, "and your strength will be spent in vain." The meaning is the same as if the Torah had written ותם כחכם לריק, meaning that your entire efforts will be to no avail. The Torah had to tell us this as the power of destructive forces often is only partially effective.