From Chabad.org: The Jews arrive at Mount Hor. At G‑d’s command, Moshe, Aaron and Aaron’s son, Elazar, go up the mountain. Aaron removes his high priest’s vestments and Elazar dons them. Aaron then passes away. The entire nation mourns Aaron’s death for thirty days. The Amalekites, disguised as Canaanites, attack the Jews. The Jews pray to G‑d and are victorious in battle. The Jews complain about their food, claiming that they are “disgusted” by the manna. G‑d dispatches serpents into the Israelite encampment, and many Jews die. Moshe prays to G‑d on the Jews’ behalf. Following G‑d’s instructions, Moshe fashions a copper serpent and places it atop a pole. The bitten Jews would look at this snake and be healed.
And why did they Amalekites disguise themselves as Canaanites? The Midrash explains that they figured that the Jews would pray to G-d to help them defeat these enemy Canaanites, and because they weren’t really Canaanites, their prayers wouldn’t work. Their mistake was that they only disguised their language, not their clothes, so when the Jews saw them, they weren’t sure who they were, and simply asked for help in defeating this enemy (general request), and that worked. This definitely puts prayer in a different perspective, a perspective we could probably use more of these days.