"I always felt, from the very beginning, that moms were the yin to the gun lobby's yang," Shannon Watts tells PEOPLE

Shannon Watts was a stay-at-home mom of five in suburban Indiana on December 14, 2012 — the day 20 children and six educators were killed by a gunman at Sandy Hook Elementary School in Newton, Conn.

The next day, Watts started a Facebook page for other moms looking to have a voice in the gun violence prevention movement. “I only had 75 Facebook friends,” says Watts, whose new book, Fight Like a Mother, is featured in this week’s issue of PEOPLE. “I really didn’t know what I was doing. I just knew something had to be done.”

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“I knew nothing about my state’s gun laws,” says Watts, who was named one of PEOPLE’s 25 Women Changing the World last fall. “I knew nothing about federal gun laws. I went online, and all I could find were basically D.C.-based think tanks run by men, a few state organizations here and there, also mostly run by men. I wanted to be a part of a badass army led by women that would take this issue head on, because we didn’t have a voice.”

The Facebook page took off “like lightning in a bottle,” and soon women from across the country were reaching out to Watts asking how they could start chapters in their home states. “Women decided they didn’t need to know everything to jump in, that we could build the plane as we flew it. We trusted that all of our expertise was enough, and that we would be able to find [more] where we had gaps. That was a winning formula for success.”

Image zoom Fight Like a Mother by Shannon Watts.

Today, the nonpartisan Moms Demand Action for Gun Sense in America is the nation’s largest grassroots organization, with more than five million members.

“I always felt, from the very beginning, that moms were the yin to the gun lobby’s yang,” she says. “The gun lobby preys on the fear of a very vocal minority that their guns will be taken away. But there are 80 million moms in this country, and they’re afraid their children will be taken away. I really felt that that was emotion that would be the most effective counter to the gun lobby.”

Not only did the issue resonate for mothers, but Moms Demand is structured to empower its volunteers to take leadership positions — and many have gone on to run for office, including Georgia’s Lucy McBath, who is now a member of Congress.

Image zoom From left: Moms Demand and Students Demand activists Liv Freundlich, Julianne Moore (Liv’s mom), Ryan Pascal, Shannon Watts and Shenee Johnson were named among PEOPLE’s 25 Women Changing the World in 2018. Julian Dufort

“There are so many organizations in this country where women do the unglamorous heavy lifting,” says Watts. “They do all the menial tasks. They make the snacks. They set up the chairs. They find the venues. They do the data entry. Then men step up and do the strategy, and all of the public speaking. Not at Moms Demand Action. The vast majority of the work, soup to nuts, is done by women.”

As for the idea that reducing gun violence, which disproportionately affects Americans — the United States is home to 82% of gun deaths worldwide — is too difficult and complex to tackle, Watts says she “can’t fathom” the idea of giving up.

“It’s not an option for parents in this country, in particular, who are afraid for their children in their schools and in their communities. It’s not just the school shootings or the mass shootings, it’s the daily gun violence in city centers and in rural communities. We have to care about all of it.”

In Fight Like a Mother, Watts also addresses the many setbacks and uphill climbs she has faced as an activist, writing that she has learned to “lose forward.”