פּירוש אויף במדבר 10:9

And you will be remembered because you did as Hashem commanded you. Additionally, the teruah is a reminder to cry out to Hashem.

וכי תבאו מלחמה, “and when you go to war;” the Torah employs a variety of words when referring to becoming involved in wars, another one being והיה כי תראנה מלחמה, “it will be when war will break out” (Pharaoh to his people in Exodus 1,10)

וכי תבאו מלחמה בארצכם, “when you go to war against an enemy who has invaded your land, etc.” Onkelos translates this verse as וארי תעלון לאגחא קרבא בארעכון, “when you go out to fight a battle in your land, etc.” If that were correct, then the letter ל as the prefix to the word מלחמה would be absent in our verse and the meaning would be “if you go to war,” or “when you go to war.” This in turn would raise the question why the Torah here uses the word תבא instead of the word תצא that is standard for describing a war fought outside the boundaries of the Land of Israel. (Compare Deut. 20,1; 21,10; 23,10) Possibly, the Torah alludes to the fact that the trumpets were to be made use of in the manner described only in a war which is defined as מלחמת חובה, an obligatory war, not an expansionary war, a מלחמת רשות. All the wars described with the introductory words: כי תצא, are wars which are not obligatory, as proven by the fact that it was possible to dodge the draft and to return home after having answered the original roll call, as elaborated upon in Deut. In our verse the Torah speaks about an obligatory war, and this is why the term יצא is not mentioned here. This is also why the Torah adds the word בארצכם, “in your land,” i.e. you have been invaded. According to Sifrey our verse speaks of wars both within our borders or without, but in either case we are the victims of aggressive neighbors. The letter ו at the end of the word תבאו is therefore quite unnecessary and may be used in lieu of the missing letter ל in front of the word מלחמה.