Recall that during the last debate Jeb Bush received the biggest applause line of his campaign when he defended his brother’s antiterror record by saying “he kept us safe.” Mr. Trump is now trying to blunt that rebuke by distorting the truth about the hijackers and the Osama bin Laden era.

Blaming George W. Bush for the 9/11 attacks is like blaming President Obama for the recession that followed the 2008 financial panic. The rise of al Qaeda had been going on for years, and its first attack on U.S. soil was its bombing of the World Trade Center in 1993. Attacks on American targets overseas followed in 1996, 1998 and 2000.

Mr. Bush didn’t take office until 2001, after the 9/11 plot was underway and some of the hijackers were already in the U.S. Mr. Trump tries to blame immigration for the presence of the hijackers, but several arrived here legally on tourist or other visas.

The real U.S. failure was of intelligence and policy imagination during the Clinton years. Despite the growing evidence, U.S. officials refused to believe that Islamist terrorism posed much of a threat to Americans. Mr. Trump could bone up on this by reading the 9/11 Commission Report. Start with Chapter 4: “Responses to Al Qaeda’s Initial Assaults.”