"We have seen growing evidence that the dangers to our country can come, not only across borders, but from violence that gathers within," Bush said on Saturday at the 9/11 memorial site in Shanksville, Pennsylvania during a ceremony to mark the anniversary of the Sept 11, 2001 attacks.
"There is little cultural overlaps between violent extremists abroad and violent extremists at home ... they are children of the same foul spirit, and it is our continuing duty to confront them."
Bush, recalling the unity of the American people after the attacks, appealed for a return to that spirit amid growing political division in the country.
"When it comes to unity of America, those days seem distant from our own," he said. "Malign force seems at work in our common life ... so much of our politics has become a naked appeal to anger, fear and resentment."
Bush and his wife Laura, as well as Vice President Kamala Harris, were attending a ceremony at the Shanksville, Pennsylvania site where United Airlines Flight 93 crashed after passengers overcame the hijackers. The plane crashed in a field, preventing another target from being hit.
"In the sacrifice of the first responders, in the mutual aid of strangers, in the solidarity of grief and grace, the actions of an enemy revealed the spirit of a people," Bush said, describing the country's reaction. "We were proud of our wounded nation."
The 9/11 attacks, which killed 3,000 people in New York, Washington and Shanksville, prompted Bush to launch a U.S.-led invasion of Afghanistan that ousted the Taliban from control in Kabul and sent al Qaeda leader Osama bin Laden into hiding.