EDMONTON, Alberta - It's as true in soccer as it is in every other sport: Defense wins championships. So far at the Women's World Cup, the United States' back line has delivered a string of excellent performances.

In three group stage games, the unit allowed just one goal, and it came in the 27th minute of the opening game against Australia. Since then, the American net has not been breached.

Even though the attack has struggled, back-to-back shutouts of Sweden and Nigeria have helped provide momentum heading into the round of 16 game Monday against Colombia.

If that means the United States is known for grinding out results instead of winning with flair, it's just fine with veteran defender Christie Rampone. The New Jersey native helped close out the win over Nigeria as a second-half substitute, becoming the oldest player ever to take the field at a Women's World Cup in the process. Before then, she had a front-row seat for her teammates' strong performances.

"It's everything, having that confidence going into the next round," said Rampone, 39. "Their communication has been great, their spacing has been good, and they're blocking shots, sliding, and disrupting the [opposing] forwards."

To be sure, there have been some nervous moments, especially when Australia's and Nigeria's speedsters broke through the American midfield. But time and again, Julie Johnston - perhaps the U.S. team's best player in the tournament so far - and Becky Sauerbrunn repelled their advances.

At other times, it has been left back Meghan Klingenberg and right back Ali Krieger who have stepped up.

Klingenberg will long be remembered for her leaping clearance of a Swedish shot to help preserve a scoreless tie. Krieger repeatedly hit the deck against Nigeria, showing no regard for other players' complaints about the rug burns caused by artificial turf.

And of course, there has been Hope Solo in net. She didn't have to make the kind of spectacular saves against Sweden and Nigeria that she did against Australia, but every time she was called upon, she made no mistake with her reflexes, footwork, and athleticism.

Solo's strong performances haven't surprised her roommate in the Americans' World Cup travels, Carli Lloyd.

"We have 100 percent faith in Hope," Lloyd said. "She has come up big in big moments, and there's no doubting her."

Colombia's free-spirited attacking style will present a new set of challenges. At the other end of the field, those questions about what it will take to get the U.S. attack firing aren't going away.

But if you're looking for reasons the Americans might still have what it takes to go all the way in the World Cup, the defense makes a strong case.