That was what U.S. President Joe Biden stressed during his speech commemorating the victims of the September 11 attacks ahead of the 20th anniversary.
The White House released a six-minute video on Friday local time.
The U.S President mourned the deaths of nearly 3-thousand people, and honored those who risked their lives to rescue, recover and rebuild.
He said the tragedy showed that America's greatest strength is unity.
"We saw a national unity bend, we learned that unity is the one thing that must never break. Unity is what makes us who we are. America at its best. To me, that's the central lesson of September 11th."
Biden is expected to visit the Pennsylvania field where one of the planes was brought down, along with two other memorial sites lower Manhattan and the Pentagon.
CNN reported that no other major speech is expected at those venues.
Global leaders too paid tribute marking the anniversary.
British Prime Minister Boris Johnson said in a video message that though the terrorists imposed the burden of grief and suffering, they failed to shake people's belief in freedom and democracy.
"They failed to drive our nations apart, or to cause us to abandon our values, or to live in permanent fear. The fact that we are coming together today - in sorrow but also in faith and resolve - demonstrates the failure of terrorism and the strength of the bonds between us."
He linked the anniversary with the recent return of Taliban rule in Afghanistan, saying such events only strengthen people's belief in liberty and democracy.
Meanwhile, President Moon Jae-in also sent his condolences to U.S. citizens through social media, saying that the shock and memories left an unerasable scar on many people.
Adding that such terror cannot ever be justified, he emphasized that South Korea would actively take part in efforts to root out terrorism as reliable allies of the U.S.
Kim Bo-kyoung, Arirang News.