A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
deu 9:1MOSES DISSUADES THEM FROM THE OPINION OF THEIR OWN RIGHTEOUSNESS. (Deu. 9:1-25)
this day--means this time. The Israelites had reached the confines of the promised land, but were obliged, to their great mortification, to return. But now they certainly were to enter it. No obstacle could prevent their possession; neither the fortified defenses of the towns, nor the resistance of the gigantic inhabitants of whom they had received from the spies so formidable a description.
cities great and fenced up to heaven--Oriental cities generally cover a much greater space than those in Europe; for the houses often stand apart with gardens and fields intervening. They are almost all surrounded with walls built of burnt or sun-dried bricks, about forty feet in height. All classes in the East, but especially the nomad tribes, in their ignorance of engineering and artillery, would have abandoned in despair the idea of an assault on a walled town, which to-day would be demolished in a few hours.
deu 9:4Speak not thou in thine heart, . . . saying, For my righteousness the Lord hath brought me in to possess this land--Moses takes special care to guard his countrymen against the vanity of supposing that their own merits had procured them the distinguished privilege. The Canaanites were a hopelessly corrupt race, and deserved extermination; but history relates many remarkable instances in which God punished corrupt and guilty nations by the instrumentality of other people as bad as themselves. It was not for the sake of the Israelites, but for His own sake, for the promise made to their pious ancestors, and in furtherance of high and comprehensive purposes of good to the world, that God was about to give them a grant of Canaan.
deu 9:7Remember, and forget not, how thou provokedst the Lord--To dislodge from their minds any presumptuous idea of their own righteousness, Moses rehearses their acts of disobedience and rebellion committed so frequently, and in circumstances of the most awful and impressive solemnity, that they had forfeited all claims to the favor of God. The candor and boldness with which he gave, and the patient submission with which the people bore, his recital of charges so discreditable to their national character, has often been appealed to as among the many evidences of the truth of this history.
deu 9:8Also in Horeb--rather, "even in Horeb," where it might have been expected they would have acted otherwise.
deu 9:12Arise, get thee down quickly from hence; for thy people . . . have corrupted themselves--With a view to humble them effectually, Moses proceeds to particularize some of the most atrocious instances of their infidelity. He begins with the impiety of the golden calf--an impiety which, while their miraculous emancipation from Egypt, the most stupendous displays of the Divine Majesty that were exhibited on the adjoining mount, and the recent ratification of the covenant by which they engaged to act as the people of God, were fresh in memory, indicated a degree of inconstancy or debasement almost incredible.
deu 9:17I took the two tables, . . . and broke them before your eyes--not in the heat of intemperate passion, but in righteous indignation, from zeal to vindicate the unsullied honor of God, and by the suggestion of His Spirit to intimate that the covenant had been broken, and the people excluded from the divine favor.
deu 9:18I fell down before the Lord--The sudden and painful reaction which this scene of pagan revelry produced on the mind of the pious and patriotic leader can be more easily imagined than described. Great and public sins call for seasons of extraordinary humiliation, and in his deep affliction for the awful apostasy, he seems to have held a miraculous fast as long as before.
deu 9:20The Lord was very angry with Aaron to have destroyed him--By allowing himself to be overborne by the tide of popular clamor, Aaron became a partaker in the guilt of idolatry and would have suffered the penalty of his sinful compliance, had not the earnest intercession of Moses on his behalf prevailed.
deu 9:21I cast the dust thereof into the brook that descended out of the mount--that is, "the smitten rock" (El Leja) which was probably contiguous to, or a part of, Sinai. It is too seldom borne in mind that though the Israelites were supplied with water from this rock when they were stationed at Rephidim (Wady Feiran), there is nothing in the Scripture narrative which should lead us to suppose that the rock was in the immediate neighborhood of that place (see on Exo 17:5). The water on this smitten rock was probably the brook that descended from the mount. The water may have flowed at the distance of many miles from the rock, as the winter torrents do now through the wadies of Arabia-PetrÃ&brvbr;a (Psa 78:15-16). And the rock may have been smitten at such a height, and at a spot bearing such a relation to the Sinaitic valleys, as to furnish in this way supplies of water to the Israelites during the journey from Horeb by the way of mount Seir and Kadesh-barnea (Deu 1:1-2). On this supposition new light is, perhaps, cast on the figurative language of the apostle, when he speaks of "the rock following" the Israelites (Co1 10:4) [WILSON, Land of the Bible].
deu 9:25Thus I fell down before the Lord forty days and forty nights, as I fell down at the first--After the enumeration of various acts of rebellion, he had mentioned the outbreak at Kadesh-barnea, which, on a superficial reading of this verse, would seem to have led Moses to a third and protracted season of humiliation. But on a comparison of this passage with Num 14:5, the subject and language of this prayer show that only the second act of intercession (Deu 9:18) is now described in fuller detail.