Dareus was pleased with the new-look defense that finally began playing up to its attacking style under new coordinator Mike Pettine.
"It surprises people, but we controlled it the way we needed to," Dareus said. "We were not going to let them push us around."
The Bills pushed back in limiting Baltimore to 15 first downs and 345 yards.
It was a stifling performance that came a week after Buffalo gave up 513 yards in a 27-20 loss to the New York Jets.
And the Bills (2-2) won despite an injury-thinned defensive backfield that opened the game missing three starters, and closed it minus Williams, who couldn't finish because of a sore back.
"These guys have answered the bell, and they played their behinds off," defensive tackle Kyle Williams said. "It doesn't get any better than this."
Alonso sealed the victory by making a diving catch to pick off Flacco for a fifth time with 57 seconds remaining. Facing third-and-8 at the Bills 48, Flacco forced a pass over the middle intended for tight end Dallas Clark. Clark and safety Da'Norris Searcy both got their hands on the ball, and Alonso came up with the interception.
Fred Jackson had 87 yards rushing, and scored on a 14-yard run for the Bills, who are off to a 2-2 start under rookie head coach Doug Marrone.
C.J. Spiller added 77 yards rushing in helping Buffalo combine for 203 yards on the ground against a defense that had allowed just 224 in its first three games.
The Ravens (2-2) took a big step back after an impressive 30-9 win over Houston last week.
"A lot of that falls on me today and just throwing the ball to the wrong team," Flacco said. "If we don't do that, then this game probably looks a little different."
What it resembled instead, was another lackluster performance by a retooled offense still searching for an identity after several offseason changes to its group of receivers.
Torrey Smith had five catches for 166 yards and a touchdown, but Flacco had difficulty finding any chemistry with his other receivers.
And the Ravens running attack struggled, even with Ray Rice returning after missing last week's game with a hip injury.
Rice finished with 17 yards rushing and backup Bernard Pierce managed just 7 against a Bills defense that gave up 182 to the Jets last week.
"We would've liked to run the ball better. We would've liked to execute a lot better," Rice said. "You get behind, you can't pound the rock. You just can't do it."
Trouble was, the Ravens couldn't get their offense on track even when the game was close in the first half. Baltimore managed just three first downs and 45 yards offense on its first seven possessions.
Flacco finished 25 of 50 for 347 yards and two touchdowns, including a 10-yarder to Marlon Brown. But he has five touchdowns and seven interceptions this season.
The five interceptions were the most by the Bills defense since it had six in a 16-13 overtime win over the New York Jets on Oct. 18, 2009.
"It feels great to win," said safety Jim Leonhard, who also had an interception. "Obviously, we've had our share of adversity, and I think we've proven through four weeks that we a room full of guys on the back end that will fight."
The Bills face a short turnaround. They travel to play at Cleveland on Thursday and could be without two hobbled running backs.
Spiller is believed to have suffered a sprained ankle Sunday. Jackson said he sprained his MCL. Both players, though, said they expect to face the Browns.
Alonso, a second-round pick out of Oregon, has four interceptions on the season. And he became the team's first rookie linebacker to have an interception in three consecutive games. ... Marrone said he expects CB Aaron Williams to be healthy to play this week. ... Ravens WR Deonte Thompson sustained a concussion after being hit along the sideline by safety Jim Leonhard. ... Ravens coach John Harbaugh said both Brown and starting CB Lardarius Webb hurt their thighs and are still having their injuries evaluated.
Information from ESPN.com's Mike Rodak and The Associated Press was used in this report.
- Jerome Boger