Jared Allen doing what he does best - sacking the quarterback. The Vikings defense contrinues to brings its A game every week. (US Presswire)

Of all the reasons for the Vikings’ surprising 4-1 start (excellent game management by QB Christian Ponder, improved offensive line play, special teams that help win games, an excellent young kicker, the return of Adrian Peterson and MVP-quality play by WR Percy Harvin), the play of the defense has to be considered No. 1.

The defense has certainly improved. In 2011 the defense was ranked 21st in total defense (yards given up), 26th against the pass and 11th against the rush. The Vikings defense improved in all three categories: they are ranked 7th in total defense (yards per game since teams have byes), 14 against the pass and 6th against the rush.

In the all-important category of points given up per game, the Vikings have improved the most--moving from 31st in 2011 to a ranking of 6th in the league. Critics may be quick to point out the Vikings’ soft schedule in the first five games, but when you consider one of the teams they played had ranked 5th in total offense and 4th in points scored last season (Detroit) and another (San Francisco) nearly made it to the Super Bowl in 2011 and is 5th in yards per game allowed, the Vikings schedule hasn’t been filled with all cupcakes.

So why the improvement? The first reason has to be the personnel changes. The pass defense has certainly benefitted from the return of cornerbacks Chris Cook and Antoine Winfield, who each played in only six and five games, respectively, last season. In addition, the rookie-additions of hard-hitting safety Harrison Smith and speedy cornerback Josh Robinson have greatly impacted the style, production and versatility of the unit.

At linebacker, the change from aging E.J. Henderson to the younger Jasper Brinkley has improved the unit greatly. Brinkley has been a good run stopper, but his coverage ability has surprised even the coaches -- especially after missing last season due to hip surgery. OLB Erin Henderson is in his second year as a starter and the experience has manifested in improved play. Veteran Chad Greenway, who has led the team in tackles for years, is second in the league in that category and having one of his best seasons as a pro.

The defensive front, which garnered all kinds of attention last season for tying for the league-lead in sacks (and DE Jared Allen leading in individual sacks), is an even better unit this season with the change of DT Letroy Guion for Remi Ayodele and a platooning system that has gotten DL Everson Griffen on the field more. The unit is tied for 9th in the sack category this season, but their improvement in run stopping has more than made up for that.

But there is another personnel change that has had an effect on the unit’s performance, and that is the installation of Alan Williams as defensive coordinator. Williams’ manner is certainly less pugnacious than former DC Fred Pagac, and his ability to listen to his players -- rookies and veterans alike -- may have had the biggest impact on the team.

“He’s done a great job of understanding what his players are comfortable with and trying to compromise and find a way that we can both get done what we need to get done--and it’s resulting in wins so far,” said DE Brian Robinson.

Coach Leslie Frazier agrees:

“Part of [the defense’s success] is Alan having a better feel for our players and what their strengths and weaknesses are, along with our players really taking some accountability as to what is being asked of them,” Frazier said. The reflection of that is the success we’re having on defense. Guys are really honing in on what they’ve been asked to do at certain parts of the game, and the reflection has been the success we’re having on defense.”

The defense is better because of new, younger players, veterans who have bought into the program and a coach who listens to the players and puts them in the best position to succeed. It doesn’t sound like rocket science, but so far it has been a good formula for success. As the schedule stiffens in the second half of the season with five games against NFC Division foes, we’ll see if it can hold up.

For more Vikings news and notes, follow Joe Oberle on Twitter @CBSVikings and @joeoberle.