Entering Denver’s Monday night AFC West showdown with the first-place Kansas City Chiefs, there are few people who hold out much hope for the Broncos’ offense. The unit is perhaps the most beleaguered in the NFL right now. (And there are some really beleaguered units out there — have you watched the 49ers?) After routing the Dallas Cowboys in Week 2, Trevor Siemian and the Broncos have scored just three touchdowns in their past four games — and in their last outing were completely shut out by the Chargers.Now Denver is 3-3, with two more games against divisional leaders looming after tonight. But here’s the interesting thing about the Broncos’ apparent unraveling: The offense may be terrible, but it’s been terrible for three years — including the 2015-16 Super Bowl title run.A closer look at the numbers reveals that the vaunted Denver defense is perhaps not getting enough share of the blame. The Bronco defense is hardly struggling, but Denver’s recent formula for winning allows very little margin for error, and this year there’s been some error.Over the previous two seasons, the Denver defense was truly dominant — No. 1 in Football Outsiders’ DVOA1Defense-adjusted value over average, explained here. for both 2015 and 2016. The Broncos also finished first in passing yards allowed, first in passing net yards allowed per attempt and fourth in total points allowed in both seasons. This despite well-documented struggles at quarterback that limited the offense’s output.While the Broncos’ defense is ranked No. 1 in total yards allowed in 2017, it’s lagging behind the 2016 and 2015 editions in many key metrics. Going into Week 8, the Broncos were ranked fourth in defensive DVOA, and they were allowing a ninth-best average of 5.5 net passing yards per attempt — worsened from league-leading marks of 5.0 in 2016 and 5.1 in 2015. They’re now allowing an average of 19.7 points per game, up from 18.5 in 2015 and 18.6 in 2016.The uptick in points per game might be due to some decline in the unit’s play in high-leverage spots. Per TruMedia, before this weekend’s games, the Broncos were ninth in rate of first downs allowed per pass attempt, down from third in 2016 and first the year before. After two straight years of far surpassing the rest of league in defensive expected points added,2EPA is a metric popularized by ESPN’s Brian Burke, capturing down-to-down effectiveness based on game situation. the Broncos’ 44.84 is a very distant second to the Jacksonville Jaguars’ 83.84 — which shows the Broncos are still great at slowing offenses but not quite as great as they’ve recently been.The other culprit is the Broncos’ lack of turnovers. They’ve forced just four turnovers through six games, which currently ranks 30th in the NFL. That’s an average of 0.67 turnovers per game, down from 1.69 in both 2016 and 2015.All these stats paint an almost-complete picture of what’s going on: The Broncos are allowing slightly more yards, first downs and points to come out of the passing game while forcing far fewer turnovers, so they’re less effective at stopping opponent drives.3For the record, the Broncos had the No. 1 per-carry run defense in 2015, fell to No. 18 in 2016 and currently rank No. 2, per Pro Football Reference. But these swings haven’t made much impact on overall defensive effectiveness because of the pass-heavy nature of the NFL — and how dominant the pass defense has been.It’s easy to point to the offense here, too. If you have an offense that can’t win the field-position battle and can’t give the defense time to breathe, it’s hard to dominate on defense. This year, Broncos opponents’ average start position is 32.5 yards out, the furthest-downfield starting position in the NFL.But again, Denver’s defense should be used to this. The previous two iterations of the Broncos’ defense weren’t helped much by the offense and special teams, either; their opponents’ average start positions from their own end zone of 29.5 yards in 2015 and 29.3 yards in 2016 ranked 30th and 23rd, respectively. The field-position woes are also inflating the Broncos’ yardage defense; they’re ranked No. 1 partly because their opponents have the shortest distances to go.Really, the Broncos’ consistency has been remarkable — this year’s dip aside — especially considering the personnel changes. Pass-rusher DeMarcus Ware and safety T.J. Ward were two of Denver’s five Pro Bowl defenders from 2015; the former has since retired, and the latter was released at the beginning of the season. But plenty of superstar talent remains, including All-Pro pass-rusher Von Miller and All-Pro cornerbacks Chris Harris and Aqib Talib — and new arrivals like free-agent DT Domata Peko have made an impact.It’s literally impossible to win when a team’s offense is shut out, as the Broncos’ was in Week 7. But if Denver’s defense can regain the slight edge it had the past two years, the team doesn’t need Siemian to perform like Peyton Manning did in 2014. It just needs Siemian to perform like Manning did in 2015, when taking care of the ball in big games and moving the ball in key spots were enough to win an NFL championship.