Musar zu Bamidbar 9:18

על פי ה' יסעו, ועל פי ה' יחנו . Immediately after this, we read that the Israelites made camp at the instruction of G–d and that they broke camp at the command of G–d. The reason that the Torah repeats these reports of how the children of Israelites behaved is to teach us that we should not make a move in life without prefacing it with a remark that indicates that we hope that what we are about to do meets with G–d's approval. Whenever we arrive at our destination or complete an undertaking we are to mention that we hope that what we accomplished meets with G–d's approval, and to thank Him for having enabled us to carry out what we set out to do. By doing this our awareness of G–d will be constant, both when we set out to do something, while we are involved in it, and when we have completed it.

The words הצר הצורר אתכם, in 10,9, are a fairly clear allusion to the evil urge, seeing there is no other foe that bothers us as much as Satan. We have further support for this view in the unusual construction in verse 9, which speaks about וכי תבואו מלחמה בארצכם, "When war will come to your land." Why is the word תבואו in the plural? We would have expected instead וכי תבוא The allusion here is to the ongoing struggle between man and the evil urge. Sometimes man gains the upper hand in this struggle, at other times the forces of Satan gain ascendancy. At any rate both are constantly involved in a state of war. This is why the Torah uses the plural here. Whenever the community is called upon to perform a collective act such as to come to an assembly or to get ready to journey on it is important that they perform those acts by dedicating themselves to fulfil G–d's commands. The sound of the trumpets then is the rallying cry that helps every Israelite concentrate on what is expected of him. Such dedication is required both at the commencement of an activity and at its conclusion, therefore the Torah repeats על פי ה' יסעו, ועל פי ה' יחנו, "they would journey at the command of G–d, and they would make camp at the command of G–d," in 9,18, whereas in 9,23, we have the same instructions in the reverse order, i.e. על פי ה' יחנו, ועל פי ה' יסעו. The Torah informed us that the departure and the arrival at a site were motivated by the same intentions, namely to carry out G–d's will.