TOKYO—The trouble started early. A step out of bounds on floor exercise here. A short landing there. Over the course of two hours on Sunday, the mistakes—some almost imperceptible, some laid bare for the world to see—kept piling up, chipping away at the aura USA Gymnastics has built over the past decade. Not even the greatest of all time was immune to the realities of a sport where perfection is unattainable. For 11 years, the Americans flirted with it, at least from a competitive standpoint, flying all over the globe, then flying back home with their suitcases stuffed with gold. It still might happen at the Tokyo Olympics. But for the first time in a long time, it appears it won’t happen without a fight. Russia pulled off a stunner in qualifying, posting a top score of 171.629, more than a full point ahead of the U.S. total 170.562. While …