Once, they were impervious to any and all running backs. For 18 games, no opponent could carry the ball effectively against the Washington Redskins as they consistently kept rushers out of the end zone and kept them from gaining significant yardage.

But in the last two weeks, something has changed. The Redskins (3-1) faced two of the NFL's elite ground teams -- Seattle and Denver -- and blinked, allowing 5.6 yards per carry in splitting those games. On Sunday, they travel to Kansas City for what may be the toughest challenge yet, facing what many consider to be the league's premier offensive line and two talented runners, Priest Holmes and Larry Johnson. Redskins coaches say corrections are in order if Washington is to recover its defensive identity.

"We've got to find a way to keep that from happening," Coach Joe Gibbs said. "Otherwise we won't be winning football games. Because this team this week [Kansas City] has played some of the best offensive performances we've ever seen."

Tatum Bell essentially won Sunday's game for Denver by foiling the defense on touchdown runs of 34 and 55 yards. Bell totaled 127 yards on 12 carries -- a 10.6-yard average -- and became the first person to reach 100 yards rushing against the Redskins since Pittsburgh's Jerome Bettis last November, ending an NFL-best streak of eight games. Only once in the preceding 19 games did an opponent score twice on the ground against Washington (Green Bay's Ahman Green in the seventh game last season), and the Redskins had not allowed a scoring run of longer than 11 yards since Gibbs and his staff took over before the 2004 season. They have already yielded four rushing touchdowns after allowing seven all of last season (tied for third-best in the NFL).

The problems began in Week 4, when Seattle star Shaun Alexander broke loose in the second half, getting the Seahawks back in the game. Twice, Alexander exposed the Redskins on the outside, including a 34-yard run that at the time was the longest the Redskins defense had allowed (no back took a carry for more than 26 yards against them in 2004). The last two games have dropped the Redskins from first to 20th in the NFL in yards allowed per carry and prompted a reexamination of this most essential element of their game.

"When you pride yourself on this," defensive tackle Joe Salave'a said, "and it's our philosophy to stop the run, it really hurts. It's embarrassing, really. When you're in this system that is built around dominating the run, it's unacceptable and it's embarrassing."

"We've had a huge drop-off in the last two weeks," defensive end Renaldo Wynn said. "It's disappointing, and it comes from two or three plays, really. That right there, personally, it hurts the pride. Some teams that can't stop the run, it's because they get out-physicaled. That's not the case with us; this is things we can correct."

Last season, the Redskins allowed only 3.1 yards per carry -- a half-yard better than anyone else in the NFL -- and the lowest in modern franchise history (since 1946). That provided the backbone for the league's third-ranked defense in 2004. The players and coaches believed that a missed assignment and poor tackling have undermined them recently.

The defense expected the plays Bell exploited Sunday, stopping them repeatedly in practice last week and even calling out the scheme on the field, but mistakes cost them. Defensive end Phillip Daniels was eye to eye with Bell when he took a pitch to the left, and, although lunging and missing, still did his job, which was to force Bell inside. But when he did so, no teammates reacted, allowing Bell to cut back to the sideline and score from 34 yards out.

"We saw that play all week and just didn't play it well in the game," Daniels said. "We had several guys that could have turned that play inside and it probably wouldn't have been a touchdown. A couple of guys played the wrong side of the blocks."

The defense anticipated the cutback play Bell took for 55 yards as well -- staking Denver to a 21-10 lead -- but was again vulnerable to the outside. Several players had a shot at Bell but missed, allowing him to sprint down the sideline. "If all 11 guys don't run to the ball like we mean it," Salave'a said, "whether it be from the east or west side of the field, then you're going to have those results when teams are stretching the ball to the sidelines and having success. That's not us, and that's got to hurt."

Kansas City will show no pity. The Chiefs are tied for fourth in rushing yards per game and tied for fourth with six rushing touchdowns, and Holmes and Johnson each has a run of 35 yards. Last season, Kansas City led the league with 31 rushing scores (seven more than anyone else). The club is rested from its bye week and has offensive lineman Willie Roaf back from injury.

"If we want to turn it around," Wynn said, "there's no other big way than to do it than against Kansas City up there where they love to run the ball. Offensively, this might be the best team we've faced thus far, even better than Seattle. It's going to be a big test."

Until the past two games, most opposing runners, such as Julius Jones of Dallas, above, had been contained. Shaun Alexander, shown evading Marcus Washington, left, and Matt Bowen, broke loose in the second half of Week 4.