ATLANTA — Mauricio Pochettino spent his first 18 months as coach of the U.S. men’s national team trying to change the culture. He insisted that there was no A-team, that nothing would be handed out. He brought in some new faces and trotted out some new slogans. He experimented with tactics, seeming to hit on something with a switch to a 3-4-3 during September camp that kept bringing results in October and November.