ONCE AGAIN FUELED by its defense, the United States women’s national team pulled out another victory in World Cup play, this time earning a well-deserved 1-0 win over Nigeria on Tuesday night in Vancouver.

On the back of a first-half goal by Abby Wambach, the U.S. has now finished atop Group D and will move on to the Round of 16 against a to-be-determined opponent. Here are three thoughts on the win.

ATTACK WAS BETTER, BUT...

With Alex Morgan earning her first start of the tournament, the U.S. offense looked alive for the first time in Canada. Morgan’s trademark pace, as well as her symbiotic relationship with strike partner Wambach, led to the U.S. creating more chances on goal than in its first two matches combined.

The bad news is that even with Morgan—and a one-player advantage for the last 20 minutes of play—the U.S. offense still only generated one goal, and that came from a set piece. Morgan herself had several opportunities to score, including three open looks in the box—all of which were saved.

Wambach, for her part, was a bit more productive than she had been against Australia in her other start, but still could have been more impactful overall. She did have two excellent headed flicks to teammates for chances on goal—one of which resulted in a semi-controversial disallowed goal by Julie Johnston. The U.S. was also unlucky not to earn a penalty on what seemed to be a clear Nigerian handball in the 34th minute.

Offside call / disallowed #USA goal. #USAvNGA pic.twitter.com/aQhyz2VMpN

— Ben Jata (@Ben_Jata) June 17, 2015

Handball in the box!! #USAvNGA #USA pic.twitter.com/x54nTl3yAh

— Ben Jata (@Ben_Jata) June 17, 2015

Tobin Heath was given the start on the wing against Nigeria, her first of the tournament, but her performance was a mixed bag. While she did have some bright moments and worked hard off the ball, she also continued her maddening tendency to get caught in possession when taking too many touches.

Megan Rapinoe created a number of chances to score, including two shots she hammered to the near post, but couldn’t get off the mark. Carli Lloyd, usually a mainstay of the U.S. offense, is now scoreless in her last 10 games.

DEFENSE SUBERB AGAIN

With the American offense still stuck in second gear, the backline once again kept the ship steady. Buttressed by excellent play from center backs Becky Sauerbrunn and Julie Johnston, the U.S. kept the Nigerian’s speedy attack in check for 90 minutes.

The play of the game—on either side of the ball—came courtesy of Johnston in the 24th minute. A lack of pressure from the U.S. midfield allowed the Nigerians to find Asisat Oshoala in behind for a breakaway. However, Johnston, making a 35-yard recovery run, came in at the last-possible moment to poke the ball away from danger.

Julie Johnston with a huge tackle https://t.co/bgD6YSz9Oz

— FanSided GIF (@FanSidedGIF) June 17, 2015

At this point, with shutouts in 11 of its last 15 games and holding its opposition scoreless for the last 243 minutes of play in this tournament, the U.S. defense is the reason the team finished in first in this difficult group.

WHAT’S NEXT?

With the victory on Tuesday night, and victory in Group D secure, the U.S. will now move on to the Round of 16. One major advantage of winning the group is that the U.S. will now avoid playing Brazil, Japan, or Canada—which are all on the other side of the knockout round bracket—until at least the finals. Of course, that does mean that the U.S. could see Germany in the semifinals. France, which finishes group play tomorrow, is still a wildcard.

Another bonus for the U.S. is that the other tournament favorites have all looked vulnerable thus far, so the U.S.' offensive woes might not be the Achilles Heel everyone assumes them to be.

The Americans next play on Monday, June 22, in Edmonton.

John D. Halloran is an American Soccer Now columnist. Follow him on Twitter.