What had Israel already done to the Amorites before arriving in Moab?
Israel had already fought battles against Amorite tribes in the Sinai region and utterly destroyed them. Word of this victory spread by mouth through camel caravans, so that by the time the children of Israel pitched in the plains of Moab, Balak king of Moab had heard and seen all that Israel had done to the Amorites.
Les explains that before Israel arrived in Moab, they had already fought one or two battles in the Sinai, including against Amorite tribes, and they just utterly destroyed them. Although there was no radio, television, telephone, or telegraph, word still spread by mouth as camel caravans traveled from Egypt up into Mesopotamia. The Moabites heard all that had happened to the children of Israel, all the way back to their exodus. Balak, the king of Moab, thus heard and saw all that Israel had done to those Amorites, and he was sore afraid of the people because they were many.