PHILADELPHIA — Everything was going the Dallas Cowboys’ way, and then nothing was.

After jumping to a three-touchdown lead, the Cowboys suddenly found themselves trailing the Philadelphia Eagles in the biggest game of the season for both teams.

“That’s when you have to show your toughness, mentally and physically,” Dallas quarterback Tony Romo said after he led the Cowboys to a 38-27 victory Sunday night at Lincoln Financial Field. “That’s when you just have to put your head down and go do your job.”

Romo did his job, throwing three touchdown passes to wide receiver Dez Bryant as Dallas moved past the Eagles into first place in NFC East.

The Cowboys (10-4) finish their season at home against the Indianapolis Colts and at the Washington Redskins, while the Eagles (9-5) finish with two road games, at Washington and at the New York Giants.

If the teams finish tied for first place, the Eagles would win the NFC East because they would have a better division record than the Cowboys. Dallas and Philadelphia split the head-to-head series.

“We’ll enjoy this one for 24 hours,” Romo said, “and then we have to let it go and get ready for the next biggest game of the year.”

The Eagles now have to hope Dallas loses one of its final two games and they win both of theirs because they know that they could finish 11-5 and still not make the playoffs if Dallas wins the division.

“It’s disappointing because we lost a game we had to win,” Eagles center Jason Kelce said.

The Cowboys jumped to a 21-0 lead and were ahead 21-10 at halftime, but the Eagles scored the first two touchdowns of the second half to take the lead.

Philadelphia got a quick start to the third quarter thanks to a 72-yard catch-and-run by wide receiver Jeremy Maclin, who beat defender Brandon Carr on an out pattern. When Carr overran the play, Maclin took off down the sideline and was finally dragged down on the Dallas 1-yard line. Running back Chris Polk ran the final yard on the next play to make it 21-17 with 8:48 left in the third.

The Eagles then got a break. Tight coverage forced Romo to hold onto the ball and he was blindsided by defensive end Vinny Curry, and he fumbled the ball away on the Cowboys’ 14. Two plays later, running back Darren Sproles scored on a 1-yard carry, and the Eagles had their first lead of the game, 21-17.

“We never gave up, we fought and fought, and our offense got us back into the game,” Philadelphia safety Malcolm Jenkins said. “But we knew the game wasn’t over then, and it wasn’t.”

The Eagles’ lead didn’t last for long. The Cowboys took the ensuing kickoff and quickly drove 78 yards in eight plays for a touchdown. A 22-yard pass to Bryant (six catches, 114 yards) put the ball on the Eagles 2. Running back DeMarco Murray (31 carries, 81 yards, two touchdowns) scored on the next play, and Dallas was back on top 28-24 with 39 seconds left in the third quarter.

“That drive was huge because they had all the momentum and the crowd was going crazy and it could have gotten away from us right there,” Dallas guard Zack Martin said.

The Cowboys’ defense then came up with a takeaway, as safety J.J. Wilcox intercepted a tipped pass from quarterback Mark Sanchez on the Eagles’ 42. That set up another big play between Romo and Bryant, who once again beat cornerback Bradley Fletcher, this time for a 25-yard touchdown that made it 35-24 with 12:48 left in the game.

After Philadelphia kicked a field goal and then stopped the Dallas offense, the Eagles made another crucial mistake. Tight end Brent Celek fumbled the ball away on the Philadelphia 34-yard line, setting up a 49-yard field goal by Dallas placekicker Dan Bailey that sealed the victory with 4:37 to play.

“We got it rolling there for a while,” Sanchez said. “We got hot, and guys were making plays. And then we started to turn the ball over.”

Romo finished 22 of 31 for 265 yards. Sanchez went 17-for-28 for 252 yards with no touchdown passes and two interceptions. Maclin caught four passes for 98 yards.

The Cowboys got a huge early break when the Eagles’ Josh Huff let the opening kickoff bounce toward Dallas’ coverage team. Safety C.J. Spillman wrestled the loose ball from Huff, and the Cowboys were in business on the Eagles 18. That led to Murray’s 1-yard touchdown run and a 7-0 lead with 12:46 left in the first quarter.

Dallas had to work a little harder for its next touchdown, driving 88 yards in 16 plays and 8:16. The key to the drive was three third-down passes to tight end Jason Witten, for 21, 17 and 4 yards. That set up Romo’s 4-yard touchdown pass to Bryant, and the Cowboys had a 14-0 lead with 3:04 left in the first quarter.

After the Eagles’ offense went three-and-out again, the Cowboys struck for a touchdown again, this time scoring on Romo’s second scoring pass to Bryant, a 26-yarder. That made it 21-0, and the Cowboys had three touchdowns before the Eagles had one first down.

Philadelphia finally got its offense going and capped an 84-yard drive with Polk’s 5-yard run that made it 21-7 with 8:32 left in the second quarter. The Eagles added a 47-yard field goal by placekicker Cody Parkey to cut Dallas’ lead to 21-10 at halftime.

What the Cowboys said:

“I know we always say that every game is the most important game of the season, but this one really was.” — Defensive end Jeremy Mincey.

What the Eagles said:

“I can’t commit two turnovers like that. I didn’t play well enough to win, and that’s why we lost.” — Quarterback Mark Sanchez.

What we learned about the Cowboys:

1. Dallas can win even when its workhorse, running back DeMarco Murray, doesn’t dominate as he has for most of the season. Murray rushed for 81 yards, but he needed 31 carries to do it. His per-rush average of 2.6 yards was his lowest of the season. However, Murray did score two touchdowns.

2. The Cowboys can win big games in December after all — and they can beat the Eagles in December, as well. Dallas had lost six of its previous seven December games against Philadelphia. Now the Cowboys have a running game to complement the Tony Romo-Dez Bryant connection, as well as a rugged defense, and those are the two things a team needs when the weather gets cold.

–K Dan Bailey has been a big part of the Cowboys’ success. He kicked a key 49-yard field goal in the fourth quarter that made it a two-score game. The fourth-year player from Oklahoma State is 111-for-123 in his career, a 90.2 percent rate that is the best in NFL history. In addition, Bailey is 35-for-38 on kicks from 40-49 yards.

–WR Dez Bryant was practically invisible when the Eagles pounded the Cowboys 33-10 on Thanksgiving, catching just four passes for 73 mostly meaningless yards. However, he had a big night in the Cowboys’ biggest game of the season, catching six passes for 114 yards and three touchdowns in the rematch against Philadelphia on Sunday. “I’m a competitor, and I’m always going to come back strong,” Bryant said. “They did a good job on defense the last time, but we knew we could make plays against them, and today we did.”

–TE Jason Witten caught one pass for 8 yards in the Cowboys’ 33-10 loss to the Eagles on Thanksgiving Day, and he wasn’t happy about it. He made up for it in the rematch Sunday night, catching seven passes for 69 yards, including several catches that picked up big first downs. “I know I’m a big part of this offense, and I didn’t show that the last time,” Witten said. “But I’ve also been doing this for a long time now, and I know that you can’t let one bad game affect you. I knew I’d get a chance to redeem myself against these guys, and this was the perfect time to do it.”

What we learned about the Eagles:

1. The secondary has been a problem for this team all season, and it was a big issue again Sunday night. Cornerback Bradley Fletcher got burned by Cowboys wide receiver Dez Bryant for three touchdowns, and cornerback Cary Williams was flagged for three penalties, two of which kept scoring drives alive. The defensive backfield will be the Eagles’ main area of focus in the offseason.

2. When the Eagles are good, they can be very good. However, when the are bad, they are really bad. Four of their five losses came against the Arizona Cardinals, Green Bay Packers, Seattle Seahawks and the Cowboys — all among the top teams in the NFL — and only once in those contests did they stay within a single-digit deficit. If the Eagles want to be an elite team, they must play better against elite teams.

–QB Mark Sanchez probably will be the starting quarterback the rest of the season, even though the 9-4 Eagles are now just 3-3 in games he started. Sanchez, who threw two interceptions in Sunday’s loss to Dallas, is the first Philadelphia QB with four multiple-interception games in a season since Randall Cunningham in 1988, and Sanchez has only played in seven games (he subbed for an injured Nick Foles vs. Houston). “I’m the first guy to say that I’m just upset with myself,” Sanchez said. “You can’t turn the ball over.”

–LB Connor Barwin now has 14 1/2 sacks and DE Vinny Curry has nine after each recorded one Sunday. As bad as the secondary has been, the Eagles’ front seven has become one of the best in the NFL. Not only did Philadelphia get good pressure much of the night, but it also held the NFL’s leading rusher, Dallas RB DeMarco Murray, to just 2.6 yards per carry.

–TE Brent Celek committed a crucial turnover Sunday night. The Eagles trailed by eight points in the fourth quarter and were driving when Celek caught a first-down pass and then fumbled the ball away even though he wasn’t hit hard. Dallas ended up kicking a clinching field goal. “I don’t really know happened,” said Celek, who caught four passes for 52 yards. “I wasn’t extending the ball or anything. I guess somebody knocked it out, but I know I can’t let that happen. That turnover killed us.”