And Moab said to the elders of Midian, “This horde will now lick up all that is around us, as the ox licks up the grass of the field.” So Balak the son of Zippor, who was king of Moab at that time...
Numbers 22:4

Old enemies will unite to oppose a new mutual enemy. "Us vs Them" can be a great cohesive force. It's not necessarily a bad thing in itself, but it is easier to use for evil purposes than for good.