Standing before the Israelites on his final day, Moses delivers a parting address to transfer the mantle of leadership to Joshua. His intention is not merely to update the nation on his condition, but to comfort them, ease their fears about his impending death, and assure them that divine providence will continue to guide them even after he is gone [רמב״ן, ספורנו, מלבי״ם]. He notes his advanced age of one hundred and twenty years. While this might be understood as a general reference to his current stage of life [העמק דבר], the primary approach among commentators is that it is meant entirely literally. It is his exact birthday. Moses was born and died on the seventh of Adar, demonstrating that God completes the years of the righteous to the very day. This precision reflects a life lived to its absolute fullest, achieving a perfect harmony between the physical and the spiritual [רש״י, תורה תמימה, חומש קה״ת].
Moses then declares his inability to continue moving with agility, going to war, and bearing the heavy burden of leadership [אבן עזרא, שד״ל, ביאור יש״ר]. This statement presents a challenge, as it is later recorded that his physical vitality and eyesight never failed him. Some explain that his natural strength had indeed diminished with age; the miraculous energy he possessed was granted by God solely for his mission, and once that mission concluded, his body returned to its natural human state [אבן עזרא, רש״ר הירש]. Others, however, maintain that he remained at peak physical condition. According to this perspective, he spoke of his decline simply to comfort the people and lessen their grief over his departure [רמב״ן]. Alternatively, his inability was not a matter of physical weakness, but of authorization. The right to lead had been officially transferred to Joshua, meaning Moses was simply no longer permitted to act as their leader [רש״י, ברטנורא].
Beyond physical or authoritative limitations, another tradition suggests this limitation relates to his mastery of Torah. Miraculously, the deep wellsprings of wisdom were sealed off from Moses, leaving him unable to answer the people's questions. This was an act of profound divine kindness. By experiencing this loss of clarity, Moses would not feel sorrow upon seeing his student Joshua assume his position, allowing him to leave the world in absolute peace rather than mourning the loss of his leadership [רש״י, שפתי כהן, צאינה וראינה].
Finally, Moses reminds the nation of God's decree preventing him from crossing the Jordan River. This can be understood as the direct cause of his inability to lead—he can no longer fulfill his duties precisely because God has forbidden him from crossing [רש״י, מזרחי]. Conversely, it can be viewed as an additional factor: beyond his age or the transfer of power, an explicit divine decree stands in his way [רא״ש, דעת זקנים]. He mentions this decree to dispel any lingering false hopes the people might harbor about his continued leadership. Yet, at the same time, it serves to instill them with deep confidence. Although Moses will not cross the river, he assures them they have nothing to fear, for God Himself will cross before them and lead them in their battles [ספורנו, חזקוני, פענח רזא].