The NFL’s biggest surprise this season — with the possible exception of the Pittsburgh Steelers’ nosedive — is the emergence of the Chicago Bears as a potential super-team. It’s not entirely a surprise that the Bears are winners. They were, after all, 11–5 last season before losing to the Carolina Panthers in the playoffs, and they had, by consensus and most key statistics, the best defense in the league. All that was necessary this year for a rebirth of the “Monsters of the Midway” was a reasonable-to-good offense.

Actually, it is something of a shock that the Bears are unbeaten. Plenty of teams over the years have led the league in key defensive stats and flopped the following year, most recently the 2005 Buffalo Bills, who many thought had the best defense the year before.

The problem in measuring the effectiveness of defenses is that it’s difficult to know how accurate the numbers are if a team doesn’t have a good offense. Which is to say, if a team isn’t much of a threat to score, their opponents are probably going to play conservatively. And knowing they don’t have to score much to get a win, opponents are less likely to risk interceptions and damaging hits on their own quarterback. You really don’t know how good a defense is until it’s pressed. That was the case with 2004–05 Bills. But in retrospect, it probably wasn’t — and isn’t — the case with the 2005–06 Bears.

First, the Bears’ head coach, Lovie Smith, has a well-earned reputation as one of the best defensive coaches in the NFL (the St. Louis Rams had great defenses when he was their defensive coordinator and lousy ones when he left). Second, the Bears defense last season was tested under fire, since the team was in playoff contention all season and had to carry not only their load, but also that of their offense, which floundered when quarterback Rex Grossman was injured and no suitable replacement could be found.

This year, so far, the Bears are clicking on all cylinders.

How good have they looked? Consider this: The most dominant NFL team since 1980 was probably the 1985 edition of the Chicago Bears. Here’s a bottom line comparison of the first five games for the ’85 Bears compared to five games with this bunch:

The numbers posted by the ’85 team probably would surprise the average fan. Those Bears had a reputation not merely for a great defensive squad but as perhaps the greatest defensive team ever. Their offense, in fact, was thought to have coasted through the season, carried by the defense. This was not the case: The 1985 Bears, like all great NFL teams, had a terrific offense. In fact, they led the league in scoring with 456 points, 45 more than the NFC’s second best team — the defending champion San Francisco ’49ers led by Joe Montana.

In 1985, Chicago’s offense was potent and multifaceted. Walter Payton was second in the league with 1551 yards and a 4.8 average, although Payton had a lot of great rushing performances in years when the Bears offense was mediocre. What tipped the scales for the Bears in ’85 was Jim McMahon, who was second in the league in yards per throw at 7.6, just a fraction behind the Giants’ Phil Simms and, second in the league overall in passer rating (outperformed only by Montana). The combination of great passing, strong running, and a defense that created scoring opportunities and provided great field position was deadly: The Bears led the NFL in points scored and points allowed, posted a 15–1 regular season record, and stormed through the playoffs to crush the New England Patriots 46–10 in the Super Bowl.

What’s scary about this year’s Bears is that they seem better than their ’85 predecessors in every facet of the game except running. Leading rusher Thomas Jones, with just 388 yards so far for a 3.7 average won’t make anyone forget Walter Payton, but with Rex Grossman tearing up opposing defenses at an alarming rate, the running game will get plenty of opportunities, and that’s what matters. At age 26, Grossman’s performance is the most eye opening of the young season. Grossman, who has been regarded as a fine passer, though no more than when he played for Florida, has stumbled through three injury-filled seasons with the Bears, throwing just 195 passes in 8 games between 2003 and 2005 with four touchdowns and six interceptions. But working closely with the Bears brilliant offensive coordinator, Ron Turner (brother of Norv), Grossman has flourished. Through the first five games, he’s thrown 10 touchdown passes and averaged 8.2 yards a throw, both on pace to surpass McMahon’s ’85 numbers (15 and 7.6).

Defensively, the 2006 Bears don’t seem to have any outstanding stars to rival the ’85 squad, which featured defensive end Richard Dent, William “The Refrigerator” Perry at tackle, and linebacker “Samurai Mike” Singletary.Middle linebacker Brian Urlacher is the best-known player on this year’s team, and even he has his detractors. But the Bears defense plays as a unit, and that’s what counts. So far, they have not only the best defense in the league but are on pace to beat the ’85 Bears’ total of points allowed, by a staggering 83.

That isn’t likely to happen, of course, but the Bears don’t have to win by such margins in order to be the best team in the league. They’re clearly that right now, and if they play anywhere near this level for the rest of the year, they’ll probably be favorites to take it all once the playoffs begin. It’s been more than a quarter of a century since any NFL team has put such a strong signal that the road to the Super Bowl goes through them.

Mr. Barra is the author of “The Last Coach: A Life of Paul ‘Bear’ Bryant.”