CARDIFF, Aug. 17 -- Enzo Maresca had been Manchester City manager for less than a minute of competitive football when everything started going wrong.

Myles Lewis-Skelly collected the ball wide on Arsenal's left flank and drove at a City backline that looked uncertain, unfamiliar, and already scrambling. His cut-back found Riccardo Calafiori arriving at the far post, and before Maresca had processed what his new side was supposed to be doing, Arsenal were ahead. The Community Shield at Principality Stadium was thirty-seven seconds old.

Arsenal went on to win 3-0, with Martin Odegaard adding two more goals in a performance that felt less like preseason housekeeping than a declaration. The defending Premier League champions did not come to...