Exertional sickling from sickle cell trait (SCT) can pose a grave risk for some military recruits and is a troubling cause of death in college athletes. We report the cases of two U.S. Army recruits with undetected SCT who collapsed and soon died from metabolic complications of exertional sickling as they struggled to finish in time the 2-mile run of the Army Physical Fitness Test, having failed this test on prior attempts. These cases are similar to other military cases and to recent sickling deaths in college track and football. Research shows how and why, in the face of SCT, during intense exercise bouts, sickle cells can quickly form and lead to fulminant rhabdomyolysis that can be fatal. Increasing evidence suggests that, in the military and in sports, the proximate trigger for most cases of fatal sickling collapse is intensity. If this hypothesis is correct, that sickling collapse is an intensity syndrome, it raises vital questions about how best to train military recruits with SCT.