The San Antonio Spurs were a historically bad defensive team last season. They had the highest defensive rating in franchise history (120.0) and gave up 123 points per game.
There were plenty of reasons for their porous defense. One that sticks out is the inability to make impact plays on the defensive end. There was little-to-no aggression on that end of the floor, with San Antonio finishing 26th in blocks (3.9) and 22nd in steals (7.0.) Strong defenses are typically good at turning the other team over, and protecting the basket.
Defense will be a point of emphasis for the Spurs next season.