Where Seattle's D ranks all time

— -- The Seattle Seahawks are built primarily around their suffocating Legion of Boom defense, and that defense has now driven them late into the postseason for the third straight year.

If the Seahawks win their second straight championship -- our simulations at Football Outsiders have them winning 37 percent of the time -- they will have won those two titles by vanquishing the best quarterbacks the rest of the NFL has to offer. Last year, they took out Drew Brees and Peyton Manning. This year, they'll have to take out Aaron Rodgers and then either Tom Brady or Andrew Luck.

So it's a worthwhile question to ask: Is this the best defense in NFL history? Or at the very least, the best in recent NFL history?

If we're only looking at a single regular season, then the clear answer is "no." This year, the Seahawks did finish the season at No. 1 in Football Outsiders' advanced DVOA ratings. (DVOA, which stands for defense-adjusted value over average, is explained further here.) But the Seahawks were one of the lowest-rated defenses to ever finish at No. 1, and certainly were not as good as they were a year ago.

We have play-by-play broken down to do DVOA ratings for 26 seasons, from 1989 through 2014. The 2014 Seahawks had a defensive rating of minus-16.3 percent, which ranks them behind 44 different defenses of the previous 25 seasons. The 2013 Seahawks, on the other hand, had a rating of minus-25.9 percent, which ranked seventh among all teams over that span.

However, what if we look at the Seahawks over this entire extended period rather than just one season?

One of the best in recent history

Over a three-year period, the DVOA ratings system definitely scores Seattle as one of the best defenses of the past 25 years. According to our numbers (see the chart below), they are the fifth-best unit over the past 25 years and the best of the past 11. But the Seahawks can't quite match up to one of the greatest -- and probably the most underrated -- defensive teams of all time, the Philadelphia Eagles of the early '90s.

The Eagles hired Buddy Ryan away from Chicago in 1986, and he built dominating defenses that continued to punish the league even after he was fired at the end of the 1990 season. The Eagles had an unstoppable pass rush built around All-Pros Reggie White, Jerome Brown and Clyde Simmons; the players behind them included cornerback Eric Allen, strong safety Andre Waters and linebacker Seth Joyner. These players continued to destroy opposing offenses even after Rich Kotite and Bud Carson took over for Ryan and the defensive coordinator who left with him, Jeff Fisher.

For seven straight seasons, from 1988 through 1994, the Eagles ranked in the top five in defensive DVOA. The pinnacle came in 1991, when the Eagles were 42.4 percent better than an average defense, according to DVOA. That's the best rating of any team since 1989, by leaps and bounds; the 2002 Tampa Bay Buccaneers are the only other team over 30 percent.

(Why have you probably not thought much about the 1991 Eagles? Their offense was so horrendous. Randall Cunningham tore his ACL four pass attempts into the season, and the Eagles used Jim McMahon, Jeff Kemp and Brad Goebel as their quarterbacks. They missed the playoffs at 10-6 because eight different NFC teams won 10 or more games. And thus, they never come up in most discussions of the greatest defenses ever.)

If we simply average out defensive DVOA over three years, the 1989-1991 Eagles come out as the best defense in recent history, followed by the 1990-1992 Eagles and the 1991-1993 Eagles. This method ranks the 2012-2014 Seahawks just 10th. However, to try to somewhat neutralize the effect of one outlier year like the one the Eagles had in 1991, we can use a method called the "harmonic mean" to look at the best defenses over three years ( explained here). With that method, the Seahawks have the fifth-best three-year string since 1989.

However, this only takes us back to 1989; what about before then?

Going back further

Last year, a Football Outsiders reader named Andreas Shepard used standard stats and strength of schedule to develop estimates of historical DVOA going all the way back to 1950. Using harmonic mean of those estimated ratings over a three-year period, the Seahawks barely make the top 20. On the new list, the title of best defense over an extended period belongs to the Purple People Eaters, the Minnesota Vikings of 1969-1971. That team is followed by the Chicago Bears of 1984-1986, and then the Pittsburgh Steelers of 1974-1976. In fact, five of the top 20 defenses over a three-year span are different iterations of the Steel Curtain dynasty between 1972 and 1978.

It's worth noting that the simple rating system (SRS)  used at Pro-Football-Reference rates the Seahawks even higher than our DVOA numbers. Using a three-year harmonic mean of SRS numbers, the 2012-2014 Seahawks score as the best three-year defense since the 1978 liberalization of passing rules. SRS still puts the 1969-1971 Vikings on top, followed by the 1965-1967 and 1964-1966 Green Bay Packers. The 2012-2014 Seahawks are fourth, and the 1984-1986 Chicago Bears fifth.

Difference between 2013 and 2014 -- and the key to Seattle repeating as champs

However, SRS numbers for defense are not only measuring Seattle's defense. They are based on points allowed, and that number is influenced by field position, which in turn is influenced by the other units on the team. And that ties into the first of two main reasons the 2014 Seattle defense was not quite as good as you might think.

Part of the reason behind Seattle's success on defense is that it's put into a good position by an equally outstanding Seattle offense. The Seahawks ranked fifth in offensive DVOA this year, but their offensive rating of 16.7 percent above average is actually slightly stronger than their defensive rating. By sustaining drives and limiting turnovers, the Seattle offense limited the number of times it forced the Seattle defense to play a short field. Therefore, the Seattle defense began its average drive with the opposing offense needing to go  a league-high 74.8 yards for a touchdown.

But when the opposing offense made up most of those yards and got down near the goal line, that's where we found the biggest difference between the 2013 and 2014 Seahawks. The Seahawks' defense wasn't good in the red zone this season. I don't mean that they were average, as opposed to being fantastic elsewhere on the field. I mean they were actually very bad. The Seahawks ranked 28th in DVOA in the red zone. They allowed 5.2 points per red zone visit by opponents; only New Orleans and Indianapolis were worse.

The good news for Seattle is that this is the weakness that got fixed when the Seahawks went on that dominating six-game winning streak to close the season. In their first 10 games, Seattle's defense was successful by our definition on a league-low 42 percent of plays. In the final six games, that number shot up to 70 percent.

(A successful play will either take the ball away from the offense or prevent the offense from gaining 45 percent of needed yards on first down, 60 percent on second down or 100 percent on third or fourth down.)

With better play in the red zone, the Seahawks are back to being the defense they were a year ago. And while their defense over a three-year period, on average, hasn't been one of the absolute best in NFL history, that defense from just 2013 certainly was. If that defense shows up this week and again in Arizona, the Seahawks will be lifting a second Lombardi Trophy.