IRVINE, Calif. — When the United States opens its World Cup campaign against Paraguay on Friday, it will do so against an opponent that helped shape a defining characteristic of Mauricio Pochettino’s team.The last time the sides matched up, it could only technically be described as a “friendly.”A tense, physical game in Philadelphia in November erupted into a fracas in stoppage time. USMNT’s Alex Freeman and Paraguay’s Gustavo Gomez wrestled over a throw-in, with Gomez aggressively trying to take the ball away.That moment — and the reflexive, visceral reaction for all American players who rushed towards danger to defend their brothers — served as a tangible moment of bonding. It showed every person in that locker room had each other’s back.“We got a lot of dogs on the team,” defender Miles Robinson told The Athletic recently. “It’s about letting ‘em out the cage.”Robinson swiftly was put in a headlock by a Paraguayan player during the melee. Pochettino ran over, slipped and was helped back up by Paraguay head coach Gustavo Alfaro.“It was a spark for the group and for the fans,” Robinson said. “It was a moment we needed as a group to recognize, like, hey, we’re not to be messed with or taken lightly. We’re in it for the long haul and in it for each other.”