ATLANTA — Inside Craig Ivey’s travel bag are objects reminiscent of the Middle Ages.

He has a steel, rounded shield; a five-sided, wooden shield; a red, white and blue surcoat; a protective vest; a wraparound helmet, pockmarked with dents; steel pads to hide his forearms, knees, legs and hands; and a blunt-edged sword designed to inflict pain but not cut. His collection cost about $4,000.

Ivey, a fitness trainer in Atlanta, will use all 60 pounds of the equipment Thursday at an outdoor arena in Aigues-Mortes, in the south of France. He will compete in his first Battle of the Nations, a modern-day, medieval-like combat involving national teams of fighters....