RIO DE JANEIRO — Two groups of men stood on opposite rooftops perched on a hillside overlooking Rio de Janeiro's Ipanema beach, taunting one another. It was a macho showdown between opponents wielding ...

As anyone who has read the best-selling novel The Kite Runner knows, springtime in Kabul is heralded by flocks of dipping, looping and diving kites. But these aren’t the kites of lazy weekend picnics.

“That’s the logic of kite flying: cutting another person’s line,” said Alexander Mattoso da Silva, a military police officer with bulging, tattooed biceps. He goes by “Jarro” and in 2014 he traveled ...