Or HaChaim on Numbers 21:1/אור החיים על במדבר כ״א:א׳

1 וילחם בישראל. He fought against Israel. The reason the Torah had to write the words בישראל, instead of עם ישראל as it did in Exodus 17,8 when Amalek is reported as attacking Israel, may be that in this instance the Israelites themselves were the cause that the Canaanites attacked them. They had sinned. The words: וישב ממנו שבי, "he took a prisoner from among them," the word ממנו suggesting that the reason Israel was attacked at this time was Israel itself. Israel's sin at this time was their share in causing the premature death of their High Priest Aaron on account of the events at "the waters of strife." Psalms 106,32 attributes G'd's anger to the people when the Psalmist exclaims: "they angered the Lord at the waters of Merivah and Moses suffered on their account." Naturally, David did not only refer to Moses but also to Aaron who shared in that mission. He singled out Moses by name as he was the principal involved.

2וישב ממנו שבי, "and took a captive from amongst them." We need to explain why the Israelites did not appear troubled by the fact that they suffered a defeat at the hands of a single Canaanite nation. We could have expected them to extrapolate that if a single Canaanite nation could inflict a defeat upon them, what would happen when they would face all seven Canaanite nations? After all, we find precisely such a reaction by Joshua when he suffered a defeat at the hands of the inhabitants of the town of Ai (Joshua 7,5-9).

3According to our sages in Bamidbar Rabbah 19,20 the people described here as "the Canaanite" were in reality the Amalekites. The reason they were described as Canaanites was that they dressed up as Canaanites in order to confuse the Israelites. This is the reason that when the Israelites had prayed to G'd to deliver the attacker into their hands they had referred to the wrong nation in their prayer. When they found out that the attackers had not belonged to the seven Canaanite nations they were not overly concerned at the minor victory enjoyed by the Aamalekite at that time. Their losses had been negligible [a single maidservant according to tradition. Ed.]. Another reason they were not overly concerned was the fact that as long as they did not try and enter the Holy Land, i.e. at a time when the measure of sin of its inhabitants was full, they did not worry that G'd would deliver them into their hands when the time was ripe. Our sages offer a variety of reasons for all this.