If I wait etc. . . Rashi adds, “If I wait,” because otherwise it implies they will surely stone him before long — whether or not he waits [to give them water].

Places this section adjacent. . . It is surprising that Rashi brings this Midrash, as he only brings Midrashim explaining adjacent sections when the sections are written out of sequence. But here they need to be adjacent because both incidents occurred in the same place. (Re”m) Gur Aryeh explains that Rashi [brings the Midrash because he] is answering the question: Why did it not say, “Amalek went out,” as in (Devarim 2:32), “Sichon went out to war against us”? Therefore Rashi explains: “The Torah places this section adjacent to the previous verse. . . that the dog (Amaleik) will come and bite you. . ..” [In other words, it says] “Amaleik came” because they asked, “Is Hashem among us or not?” [Another explanation:] I wrote many times that Rashi only explains why verses are adjacent when there are extra words. Therefore Rashi comments here about the juxtaposition. Rashi is answering the question: Is “In Rephidim” not superfluous? It is written in v. 17:1, “They camped in Rephidim.” And it is written in 19:2, “They journeyed from Rephidim.” Consequently, whatever happened in between occurred in Rephidim. Perforce, “In Rephidim” is written here to tell us that the incident occurred in Rephidim [i.e., the verse of, “They tested Hashem, saying, ‘Is Hashem among us or not?’”] caused that “Amaleik came.” And that is why Rashi says, “Adjacent to the previous verse,” rather than, “Adjacent to the previous section.” Rashi is explaining the juxtaposition [of “Amaleik came”] specifically regarding this verse. (Nachalas Yaakov)

At the time of battle, I will stand. [Rashi knows this] because otherwise, why does it say specifically, “Tomorrow [I will stand”]? Perforce, because that is the time of battle. Rashi goes on to say, “I will stand,” to convey that “tomorrow” is connected to “I will stand.” Accordingly, the verse means: Tomorrow, at the time of battle, I will stand and pray for you. This was to encourage Yehoshua and strengthen him for battle, for he will be confident of victory due to Moshe’s prayers during the battle. But if “tomorrow” refers to the preceding, [and the verse reads: “Tomorrow go out to battle Amaleik,”] then what does [“I will stand”] come to teach us?

Moshe was. . . hands were with faithfulness. Rashi is answering the question: [If ויהי refers to Moshe’s hands] why did it not say ויהיו , [the plural form, rather than ויהי ]? Thus Rashi explains that ויהי refers to Moshe, not to his hands.

Faithful and well-intended prayer. [ תפלה נכונה ] means: focused prayer. [Maharshal]

And left only. . . Rashi is answering the question: Why is it written, “Yehoshua weakened. . . with the edge of the sword”? If Yehoshua killed with the sword, then they were not weakened but killed. Therefore Rashi explains, “And left only the weak. . .”

From this we may conclude. . . Meaning: Hashem’s command was not mentioned at the beginning of this war. Rather, it is written “Moshe said to Yehoshua” (v. 9), as if Moshe commanded him on his own. Thus Rashi explains that Moshe acted according to Hashem’s command. And the proof is from our verse, for if it was not according to Hashem’s specific command, then Yehoshua [who was able to kill the mighty ones] would not have left the weak ones alive, but he would have killed them all.

The hand of Hashem. . . Rashi is resolving several issues. First, he says, “The hand of Hashem,” because the unidentified hand mentioned in the verse seems to mean Moshe’s hand. Second, he says, “Is raised,” to convey that Hashem’s hand is not always on His Throne [as the verse seems to mean]. Rather, His hand is momentarily raised toward it, similar to: “For I shall raise My hand to Heaven. . .” (Devarim 32:40). (See Kitzur Mizrachi.)