Or HaChaim on Deuteronomy 25:18/אור החיים על דברים כ״ה:י״ח

1 אשר קרך בדרך, "who happened to you on the way, etc." This expression may be understood on the basis of Rosh Hashanah 3 where we are told that the cloud of G'd was continuously enveloping the Israelites so that no one knew the exact whereabouts of this people until the death of Aaron when this cloud dissipated. At that time the Canaanite (Amalekites dressed up as Canaanites) attacked the Israelites. In view of this, how did the Amalekites locate the encampment of the Israelites? To answer this question the Torah writes אשר קרך בדרך, "who chanced upon you on the way." The Torah goes on with ויזנב בך, "and he struck your hindmost ones, the ones who trailed behind." The Torah means that the cloud did not permit the Amalekites to penetrate into the camp of the Israelites; however, they struck only those whom the cloud had expelled as unfit for this kind of protection. When the Torah speaks of הנחשלים אחריך, "the weaklings behind you," it refers to the spiritually weak Israelites.

2ואתה עיף ויגע, "and you were tired and exhausted." You were still עיף from your long and hard bondage in Egypt, and יגע exhausted from your trek through the desert.

3ולא ירא אלוקים, "and he did not fear G'd." This fact inspired more fear in you than the combined fatigue and exhaustion you had experienced. We know from Isaiah 33,14: "the sinners in Zion are frightened," that people who are aware of having sinned will experience fear as opposed to those who are serene in their knowledge of leading a blameless life. The Torah is at pains to describe Amalek in terms which will cause us to hate him by describing his surprise attack on the Jewish people, his exploiting a time when the Israelites were tired and exhausted, pouncing upon them like an eagle. At that time they were also deprived of the power of the righteous who ensured that through their conduct G'd's cloud would completely envelop and protect them.