It’s not easy to keep an elite defense in the NFL over a span of multiple years. It’s even harder to be the elite defense.

Being the top-ranked defense in consecutive seasons has only happened twice since 2000 when looking at Adjusted Defensive Net Expected Points (NEP) per play -- the 2003 and 2004 Baltimore Ravens and the 2015 and 2016 Denver Broncos.

Those Ravens were almost number one in 2000 before dropping out of the top five until 2003. They also dropped out of the top five in 2005 before taking the top spot again in 2006. They followed the same path over the next two seasons, that being outside the top five in 2007 before getting themselves back on top in 2008.

The 2017 Broncos are on pace to do something those Ravens teams couldn’t. It's not just that they're on pace to have a top-five defense for the third straight year -- it's the fact that they could be the league's best defense during this period of time.

Holding On Top

Through four weeks of play, the Broncos are the best defense when looking Adjusted Defensive NEP per play, with a slight .004 advantage over the surprising Detroit Lions. It wouldn't have been outrageous to think that Denver may take a step back in 2017 heading into this year, though. Not just because of natural regression, but also the loss of defensive coordinator Wade Phillips, who took over in 2015 when the unit went from 11th to 1st in his first season.

Some of that regression hit last year, but it didn’t completely derail the defense as a whole. They finished as the best defense in the league by NEP, but it wasn't in the all-dominating way of the 2015 squad. During that campaign, Denver ranked first against the pass by a wide margin and fifth against the rush, per our metrics. The secondary did more of the same in 2016, but the run defense dropped all the way down to 28th.

However, since the Broncos had such a big lead in the passing game -- their lead over the 2nd-ranked Philadelphia Eagles was bigger than the gap between the Eagles and the 20th-ranked Buffalo Bills -- and because passing is more important to the modern game, they were still considered the best overall defense by our metrics. The struggles in the run game, though, were a big reason why the defense couldn't pull a below-average offense -- 25th in Adjusted NEP per play -- to the playoffs.

Stopping the Run

Denver's pass defense is once again among the league's best so far this season (they rank fifth), while they've morphed back into one of the most dominant units against the run. This group ranks second under the guide of new head coach Vance Joseph and defensive coordinator Joe Woods.

Their opponents have racked up just 203 rushing yards in 2017, with none of them gaining more than 75 yards in a single game. That includes stellar performances against last year’s top rushing attacks in the Dallas Cowboys and Bills.

The Broncos held opponents at or below 75 rushing yards just twice last year, though they did have six games with an opponent held under 100. The 2016 squad also had two individual games where opponents rushed for more yards than the 2017 defense has given up in the first four games combined.

Perhaps the most impressive part about this turnaround is that it’s truly a group effort. Per NFL Game Stats and Information System, three players are tied for the team lead with eight tackles against the run: safety Darian Stewart, linebacker Shaquil Barrett, and second-year defensive lineman Adam Gotsis. Three other Broncos -- Brandon Marshall, Von Miller, and Todd Davis -- are close behind with six rushing tackles.

Upon returning from their Week 5 bye, the Broncos will face the New York Giants in Week 6, a team that currently ranks 30th in Adjusted Rushing NEP per play on offense. However, Denver’s next three games following that matchup will put this newfound success of the run defense to the test. Those all come on the road against the Los Angeles Chargers, Kansas City Chiefs, and Eagles, teams that rank 12th, 11th, and 1st in Adjusted Rushing NEP per play, respectively.

Keep Getting Better

A great run defense on early downs only helps set up the pass defense later. So far this year, the Broncos' opponents have faced 23 third downs with seven or more yards to go. While that’s only tied for the 21st-most plays in the league, the opposition has converted just 8.7 percent for a first down. The next-best defense belongs to the Pittsburgh Steelers at 10.7 percent, with the league average in those situations currently at 25.2 percent.

Denver’s pass defense might also be in for some positive regression through the rest of the season, too. The secondary has been great despite more than doubling its touchdown rate from last year. They've allowed a touchdown on 5.6 percent of opposing attempts thus far, which is the fourth-highest rate in the league after finishing the past two years at 2.4 percent (lowest) and 3.3 percent (fourth-lowest). It’s unlikely with their talent at cornerback that the touchdown barrage will continue at the same rate as last year's injury-riddled Green Bay Packers secondary.

The Broncos owned 2016's best defense while being well below the league average in one aspect. Through a quarter of the 2017 season, they're again the league’s best defense and have been among the best at everything. The Chiefs are obviously the story of the AFC West, but if Denver can continue on this path, there could be more competition at the top of that division than it would appear at the moment.