Central Iowans are being warned in advance not to be frightened by all of the rifle-toting soldiers, helicopters and humvees they may see next week. Hundreds of Iowa National Guard troops from a Sioux City-based unit are taking part in a training exercise in the Des Moines metro area.Guard spokesman Colonel Robert King says the 500 soldiers from the 113th Cavalry division will be practicing their response to simulated terrorist threats. They’ll be putting “security perimeters” around places like the Des Moines Water Works, Moffitt Reservoir in West Des Moines and Camp Dodge.Colonel King says the Guard is putting out the word of next week’s exercise now so central Iowans won’t be alarmed as the state’s military flexes its muscles in a very public way. The most visible parts of the exercise will be next Tuesday through Thursday (January 15-17).