- Georgia Stanway (29' Pen)
Stanway shrugged off the miss, tucking her second into the far corner to the delight of the largely pro-England crowd -- and coach Sarina Wiegman, who pumped a fist in celebration. All eight World Cup games so far have featured penalties.
Sarina Wiegman's England are ranked fourth -- 49 spots above fledgling Haiti -- and are considered among the World Cup favourites as the reigning European champions.
"The most important thing to come out of it is the three points," Stanway told reporters after the final whistle.
"It's so important to win your first game, going into a tournament. It's been a long build-up to today, and I think we're kind of happy to just get over the line."
England almost had a penalty earlier in the game, when Chloe Kelly was taken down in the box by Dayana Pierre-Louis -- TV cameras showing what looked like a raking stud mark down her shin. VAR, however, spotted Alessia Russo foul an opponent in the build-up.
England's best result in five previous appearances in the global showcase was a third placed finish in 2015.
England had numerous excellent scoring chances including Lauren Hemp's header late in the first half that sailed just wide of the post.
The European champions dominated the second half, but were either thwarted by the five-foot-four Theus -- who leapt high to tip a Russo header over the bar -- or their own profligacy, such as captain Millie Bright's shot that flew well over the bar.
"It was a very hard game, I'm very happy with the three points," Wiegman told ITV.
"They were very unpredictable, very transitional... And we struggled with that. We want to finish our chances. We did it with the penalty which was good because the win is the most important thing."
- Emikar Caldera