This portion includes some famous lines, e.g.:
“Justice, justice you shall pursue,” (Deuteronomy/Devarim 16:20) and
“…eye for eye, tooth for tooth,…” (19:21).
It also includes less-known teachings that have had long and far-reaching influence on Judaism:
—instructions for a king (17:14-20), including proscription of abundant horses and wives and requirement for daily study;
—instructions for just warfare (20:1-20); and
—use of a red heifer for the ritual for the unidentified murder victim (21:1-9), for example.
Less famously, this portion also presents the odd little injunction — “You must not go back that way again” (17:16) — which Moses attributes to God, although it was not previously included in God’s words.
Alter cites contemporary Israeli scholar David Cohen-Zemach suggesting that Moses refers to Exodus/Shemot 13:17:
Now when Pharaoh let the people go, God did not lead them by way of the land of the Philistines, although it was nearer; for God said, “The people may have a change of heart when they see war, and return to Egypt.”
Of course, the people often complained of losing what they had in Egypt — graves, leeks, fleshpots — but never mention horses. It is interesting to speculate on whether this might have reflected private dialogue between Moses and God, been Moses’ own admonition to the people, based on his knowledge of them and fears for their future — or perhaps a worry specifically associated with rulers. (See Abraham Joshua Heschel’s Heavenly Torah for more on if/how Moses, or any prophet, added to or interpreted God’s word.)
Plaut takes a historical view of the verse, pointing out that this was interpreted by early rabbis (Jerusalem Talmud, tractate Sanhedrin) to refer only to permanent settlement in Egypt and not to commercial trade.
Profitable as these explorations are, it can be helpful, I think, to simply read the line on its own and consider how it applies in our own lives:
“You must not go back that way again.”
See Source Materials for citations for Alter and Plaut commentaries and bibliographic information for Heschel’s work.
"A Song Every Day" 