Blake Coleman scored, and Dustin Wolf made 20 saves for the Flames (24-18-7), who have lost two straight following a three-game winning streak.
"I thought it was a pretty good game," Coleman said. "Honestly, I felt like we deserved better. Just some critical situations was the difference. I think at 5-on-5, I don't feel like it was even that close who the better team was. We just didn't find ways to put pucks in the net. A few breakdowns and mental mistakes."
Dubois put Washington up 1-0 at 1:51 of the first period with a wrist shot from the hashmarks over Wolf's glove after a turnover into the slot by MacKenzie Weegar.
"I don't know what the stats are but the team that usually scores the first goal gets a pretty big advantage on winning the game," Dubois said. "You want to score first, but there's a lot of hockey left and we did a good job of closing it off."
Coleman tied it 1-1 at 12:17 of the second period when he tipped Tyson Barrie's point shot by Thompson's blocker on a power play created by a Dubois minor for unsportsmanlike conduct.
Strome gave the Capitals a 2-1 lead at 15:12. He backhanded a shot by Wolf's glove on a 5-on-3 power play with Coleman in the box for closing his hand on the puck during Calgary's kill of Brayden Pachal's initial minor for hooking.
"Fluke thing," Coleman said. "I catch it with my top hand, which is kind of an uncomfortable spot to be in. Just a little mental mistake, wasn't sure what to do with the puck really. It puts us on a 5-on-3 and they capitalize. Can't give teams with good power plays that opportunity."