The Washington Post’s mike Jones breaks down the Redksins’ win over the Dallas Cowboys last night for the team’s first division title since the 1999 season. (The Washington Post)

A midseason turnaround that no one saw coming culminated Sunday night for the Washington Redskins with a 28-18 victory over the Dallas Cowboys that gave them an improbable NFC East title and a spot in the playoffs for the first time since 2007.

Quarterback Robert Griffin III wasn’t at his dazzling best in the win at FedEx Field. But the Redskins used the bullish running of another reliable rookie, tailback Alfred Morris, and leaned heavily on their defense to churn out a workmanlike triumph over Dallas.

The Redskins improved to 10-6 and ended the regular season on a seven-game winning streak. They seized their first division crown since the 1999 season and, as the NFC’s fourth seed for the postseason, will host the fifth-seeded Seattle Seahawks in a first-round playoff game next Sunday at 4:30 p.m.

“The main thing for our football team is they’re not satisfied with where they’re at,” Redskins Coach Mike Shanahan said. “That was our first goal.”

The Cowboys (8-8) missed the playoffs, losing a winner-take-all game for the NFC East title for a second straight season. Dallas lost to the New York Giants on the final day of the 2011 season and fell short in similarly agonizing fashion Sunday.

The Cowboys could have knocked the Redskins from the playoffs entirely. The Redskins’ wild-card chances were extinguished earlier Sunday. Instead, the Redskins will have their chance to follow in the footsteps of last season’s Giants and the Green Bay Packers of 2010, who turned hot regular season finishes into Super Bowl victories.

Griffin had a third-quarter touchdown run to put the Redskins in front for good. But it was, otherwise, a quiet game for him. He threw for only 100 yards and ran for 63 more. Morris did the heavy lifting on offense, running for 200 yards and three touchdowns on 33 laborious carries. He set the Redskins’ single-season record for rushing yards and helped them overcome a missed field goal in the first half by their previously perfect place kicker, Kai Forbath.

“I’m just thankful I was able to come out today, give it my all,” Morris said. “The run game was working today. I’m just happy we’re going to the playoffs.”

The Redskins’ defense, so heavily criticized for much of the season, did more than its share. Cowboys quarterback Tony Romo threw three interceptions, two in the first quarter and another — by Redskins linebacker Rob Jackson — with the game on the line in the final minutes.

“We had a great game plan and we played it to perfection,” Redskins defensive end Stephen Bowen said. “We forced Tony to make some mistakes.”

Romo gave the Cowboys the lead with a second-quarter touchdown pass to tight end Jason Witten. But the Redskins scored the game’s next 14 points before a 48-yard field goal by Dallas’s Dan Bailey in the opening minute of the fourth quarter.

Morris answered with a 32-yard touchdown run to increase the lead to 21-10 with about 101 / 2 minutes to play and the Redskins held on from there. Romo threw a 10-yard touchdown pass to wide receiver Kevin Ogletree, set up by Dwayne Harris’s 39-yard punt return, and added a two-point conversion on a pass to Harris with 5 minutes 50 seconds left.

The Cowboys sacked Griffin to force a punt. But Jackson grabbed a lob by Romo that was headed for Dallas tailback DeMarco Murray. That set up Morris’s one-yard touchdown plunge with 1:09 to go, after a roughing-the-passer penalty on Dallas defensive tackle Jason Hatcher kept the Redskins’ drive going. Morris reached the ball across the goal line just before it was knocked from his hands.

Romo’s big-game mettle has been questioned regularly in recent seasons, and his two early interceptions Sunday night seemed to put him in position to be blamed for another Cowboys disappointment. Dallas took over at the Redskins 27-yard line for its opening possession of the game after a 28-yard punt return by Harris. But on third down from the 25, Romo’s throw sailed high over the head of Ogletree and landed in the hands of Redskins rookie cornerback Richard Crawford for Romo’s first interception of the evening.

Romo’s second gaffe came the next time Dallas had the ball. The Cowboys drove near midfield, but Romo’s deep throw for wideout Miles Austin fell short and was intercepted by Redskins cornerback Josh Wilson.

The Redskins were faring no better on offense, though. Morris was finding running room en route to amassing 92 first-half rushing yards, but Griffin and his receivers weren’t operating at their usual efficiency. Even Forbath, who’d connected on his first 17 field goal attempts with the team, showed he’s fallible, sending a 37-yard kick off the right upright to end the Redskins’ second possession.

The Cowboys broke through in the second quarter when Romo teamed with Witten for a nine-yard touchdown. The Redskins sent only three pass rushers after Romo on a third-and-goal play. Romo held the ball for what seemed like an eternity in the pocket, then moved forward and zipped a pass to Witten in the middle of the end zone near Redskins linebacker Perry Riley. The Redskins were left pleading in vain with the officials that the play clock had expired before Dallas snapped the ball and the Cowboys should have been given a delay-of-game penalty that would have nullified the touchdown.

The Redskins had an immediate response, driving to Morris’s 17-yard touchdown dash.

They moved ahead in the third quarter on Griffin’s 10-yard touchdown run. He faked a handoff to Morris, sprinted to his left and used a block by wide receiver Pierre Garcon to get into the corner of the end zone. The Cowboys moved to the Redskins 30-yard line but their drive stalled there, and they had to settle for Bailey’s field goal. Morris followed with his second touchdown, taking a pitchout to his left and then cutting back to his right to find a seam in the Dallas defense.