When world leaders meet with Iranian leaders in Istanbul Friday, topic No. 1, of course, will be Iran’s nuclear program. But a variety of present and former security officials, members of Congress and others say the United States and other western nations are paying too little attention to a problem of near-equal gravity: Iran’s open support of terror and criminal groups all over the world. Not since the fall of the Soviet Union has any nation so aggressively pursued alliances with bad actors in so many places worldwide, acknowledged Reuel Marc Gerecht, a former CIA officer who focused primarily on...