Or HaChaim on Deuteronomy 11:8/אור החיים על דברים י״א:ח׳

1 ושמרתם את כל המצוה..למען תחזקו "You shall keep all the commandment in order for you to be strong, etc." Moses advances this argument against the people who suggested a temporary neglect of Torah study, something which physically weakens those who pursue it with intensity. The argument was that they had to husband their physical strength in order for them to conquer the Canaanites. Compare Sanhedrin 26 describing Torah study as a physically debilitating experience based on the word תושיה in Isaiah 28,29. I have mentioned previously that whenever Moses uses the expression כל המצוה in the singular he refers to Torah study. The reason Moses used the expression ושמרתם here, an expression associated with the observance of negative commandments is that neglect of Torah study would be considered a violation equal to transgressing a negative commandment. Moses concludes our verse by saying: "in order that you will be strong" to tell you that far from weakening you, Torah study and מצוה-performance will strengthen you in the forthcoming war by G'd. This is also the meaning of Proverbs 8,14: אני בינה לי גבורה, "once I have acquired insight (through Torah study) courage and strength will be mine."