Commento su Numeri 21:17

עלי באר COME UP, WELL from the valley and bring up what you have to bring up! — Whence do we know that it was the well that announced to them these miracles as it is stated above? Because it is said: ומשם בארה “and from there to the well”. For you cannot say that it means: “from that place was (they got) the well”, because was it indeed from there that they got it? Had it not been with them from the beginning of the forty years’ wanderings? But it means that from there it flowed down to the Israelites to proclaim the miracles! Then again, also, the paragraph beginning with the words: אז ישיר … השירה הזאת were spoken at the end of the forty years whilst the well was given them at the beginning of the forty, and what reason then can there be to write it as late as here. But the subject of the song has to be explained in connection with what precedes it (that it was a summons to the well to bring up the bodies of the slain) (Midrash Tanchuma, Chukat 20).

אז ישיר ישראל, Then Israel sang, etc. What precisely was the point of this song? Besides, why had the Israelites not acknowledged the mannah by a song just as they acknowledged the water? The entire paragraph needs explaining. Perhaps the entire song really was an acknowledgement of the Torah. This is why one cannot criticise that generation for not breaking out in a song of thanksgiving when the Torah was described as its מורשה, something precious left to them as an ongoing possession (Deut. 33,4). The reason is that the people had already acknowledged the gift of the Torah in the song recorded here, the song acknowledging "water." After all, Torah has frequently been compared to a well of water. It is called "well" because it originates with G'd the ultimate well from which all springs forth. It is also called "water" as it symbolises water and its life-giving properties. When the people sang עלי באר, "arise o well," this was not a reference to the physical well and the waters beneath the earth's surface, but to a celestial well. The words ענו לה are similar to Exodus 15,23 ותען להם מרים, the responsive nature of the chant.

אז ישיר ישראל את השירה הזאת, “Then Israel sang this song:” According to the plain meaning of the text the principal content of this song is the reference to the Emorites who drowned in the river Arnon and whose blood mingled with the waters from the well which accompanied the Israelites on their journeys (compare Rashi). This might account for the mention of ואת והב בסופה ואת הנחלים ארנון, “and the gift of [the Sea of Reeds and the rivers of Arnon”] (verse 14).
Personally, I believe the meaning of the verse is that what occurred there was also recorded in the Book of the wars G’d fought on behalf of the Israelites, and the word בספר in verse 14 does not mean “book” so much as “enumeration;” i.e. this “war” which the Israelites only learned about after they saw all the blood in the river Arnon was accounted another one of the wars recorded for posterity. G’d had destroyed a town known as והב by means of a tornado or something like it, סופה being a reference to a great storm. The tributaries of the river Arnon were also destroyed by that storm.
A Midrashic approach based on Tanchuma Chukat 20: the words את והב בסופה mean “what He had given to them (the miracles G’d performed, i.e. the assistance He had given) at the Sea of Reeds.” The words ואת הנחלים ארנון mean “and the rivers and the miracles He performed for them at the river Arnon and its tributaries.” What precisely were the miracles performed at Arnon? Answer: אשד נחלים “so much blood was poured into these rivers that they looked red.” Onkelos uses the word שפך here for the spilling of the blood of the Emorites, a similar expression he used in Leviticus 4,12 on the words אל שפך הדשן, which he renders as מישד קיטמא, a place for shedding blood.
The background to all this is the tradition that the Emorites were hiding in the clefts of these high rocks on either side of the river Arnon. The river was narrow but deep, its banks forming a deep canyon. The opposite sides of the rocks were so close together that people on one side could converse with those on the opposite side. However, the only way they could get across to one another was by first descending to the river and then climbing the cliffs on the opposite bank. The Emorites planned to exploit this phenomenon to ambush the Israelites when the latter would attempt to cross the river by bombarding them from above with a hail of all kinds of missives, rocks, burning torches, etc. G’d brought about a storm or earthquake which banged together the two opposing sides of these cliffs squashing he Emorites who had been hiding inside them. The blood of these people drenched the river so that the Israelites did not even become aware of their miraculous escape until hey saw that the river had become red with blood. This was the comparison with what happened at the Sea of Reeds that our verses alluded to in poetical form. The words אשר נטה, “when it veered,” in verse 15, describes the motion of the cliff in the opposite direction toward Moav during this happening. The cliff then stuck to its opposite number across the river.