SAN DIEGO — Until this week, Invisible Children seemed to be an aptly named organization. Its work in Africa to end the use of children soldiers was barely visible to most Americans.

Then the nonprofit group posted a video that went viral with help from celebrities it targeted to spread the word online. By Thursday, only a few days after its release, the 30-minute video had more than 40 million views on YouTube. Invisible Children was among the top 10 terms trending on Twitter in the United States.

Ben Keesey, the group’s 28-year-old chief executive officer, rattles off the names of celebrities who lent support when asked to explain the success of the video targeting the Lord’s Resistance Army and its leader, Joseph Kony, a bush fighter wanted by the International Criminal Court for crimes against humanity.

The website prominently displays the faces 20 disparate celebrities — from Warren Buffett and Bill O’Reilly to Tim Tebow and Stephen Colbert — asking viewers to click on them to send a message. It also shows 12 politicians from across the ideological spectrum.

“It’s something we can all agree on regardless of your political background,” Keesey said. “The core message is just to show that there are few times where problems are black and white. There’s lots of complicated stuff in the world, but Joseph Kony and what he’s doing is black and white.”

Invisible Children’s critics say the San Diego-based group oversimplifies things. In a rebuttal posted on its website to address that point and other criticisms, the group acknowledges the video overlooks many nuances but that it sought to explain the 26-year-old conflict “in an easily understandable format.” It called the film a “first entry point.”

Celebrities quickly joined the cause.

“Even if its 10 minutes … Trust me, you NEED to know about this!” Rhianna tweeted.

“This is not a joke. This serious. TOGETHER we can #MakeAChange and #STOPKRONY — help another kid in need!” Justin Bieber tweeted.

Ryan Seacrest weighed in: “watched in bed, was blown away.”

Oprah Winfrey reiterated her support for Invisible Children. “Have supported with $’s and voice and will not stop.”

Keesey said all celebrities acted without being contacted first by Invisible Children, except Winfrey.

“They all saw it online and were asked by thousands of young people via Twitter. It was all organic. They found it on their own,” he said.

Social media experts were at a loss to explain how a long, often-overlooked conflict caught the public’s imagination so quickly. Natural disasters like the Haiti earthquake in 2010 drew celebrity interest, but those stories were already making headlines worldwide.

“A lot ends up being almost a coincidence. It’s the right place right time, right story, right people. It’s rather unpredictable. If you left one of these celebrities out, it might have scotched the whole thing,” said Steve Jones, communications professor at the University of Illinois, Chicago.

The video had fewer than 10 million views on YouTube on Tuesday, when its popularity soared. It is most popular with boys and girls ages 13 to 17 and men ages 18 to 24.

Invisible Children was founded as a nonprofit organization in 2006 and operates on the fourth floor of a nondescript office tower in downtown San Diego, with about 40 full-time employees. It sends “roadies” to speak at college campuses and churches throughout the United States and Canada.

The office was bustling Thursday with 20-somethings as phones rang constantly. There were stacks of T-shirts and posters for their campaign. Walls were adorned with posters of Kony and child soldiers.

It chose its headquarters location because that is where its founders grew up, including Jason Russell, a graduate of the University of Southern California film school who narrates the video. Russell acknowledges the power of social media with the video’s first words.

“Right now there are more people on Facebook than there were on the planet 200 years ago,” he says.