Their shadow [in Hebrew, 'tzilam'] is removed from them (Num. 14:9)

Their shadow is removed from them - that is, their shield and their strength [are departed from them]. The worthy ones among them have died. Another interpretation: the shade [protection] of G‑d is departed from them. (Rashi)

It is possible that Scripture is alluding to the well-known fact that there will be no shadow over the head of a person who is [destined] to die that year, on "the night of the seal".

This is a term signifying the night of Hoshana Rabbah when the "Heavenly seal" is put upon the judgment which was decided on the New Year and Day of Atonement for each individual and his fate in the coming year.

No nation falls unless its prince falls first….

Therefore it says, "their 'shadow' is already removed from them," meaning that death has been decreed upon them - "and the Eternal is with us," for it is He Who dwells in our midst and does miracles and wonders for us in the eyes of all who behold us; therefore, "fear them not."

Or it is possible that the verse alludes to the [heavenly] princes above, for no nation falls unless its prince falls first, as it is written, "The Eternal will punish the host of the high heaven on high…" (Isaiah 24:21) and afterwards, "…on the kings of the earth upon the earth," and as is explained in the Book of Daniel. Thus, the verse is saying, "the power under whose protection the nations [in the land of Canaan] live is already removed, and 'the Eternal' Who lowered them 'is with us', therefore 'fear them not'." And thus the Rabbis said in Midrash Shir Hashirim: "And the shadows flee away": These are the princes of the nations and their angels, for they are the protection over the nations.

[Adapted from Rabbi Dr. Charles Chavel's annotated translation.]