They have the heft of an ax, a blade nearly as long as a sword, and the intimidation power to make the hair stand up on the back of your neck. Cheap and easily bought, machetes in America have commonly been reserved for underbrush and sugar-cane cutting. But now, in a spreading trend that so far has drawn little national attention, criminals are using machetes as weapons, striking fear in cities and towns across the country. Witness these recent incidents: In the heartland Indiana city of Evansville in February, a robber pulled a machete on a convenience-store cashier, who put...