Trump started the whole thing with a tautology: 9/11 happened on George W. Bush's watch. "When you talk about George Bush — I mean, say what you want, the World Trade Center came down during his time," Trump said Friday.

Jeb Bush wants the 2016 campaign to be about the future and faster economic growth. He does not want it to be about his brother's presidency, 9/11 and the war in Iraq.

The real estate mogul set a trap that Bush could not escape. Let the remarks slide and Bush would look weak. But by engaging, Bush immediately turned the 2016 campaign back to the past and to his biggest weakness: his famous last name.

On the merits, Trump 's comments are laughable. But they are also brilliant politics.

Donald Trump spent the weekend taunting Jeb Bush over the Sept. 11 terrorist attacks and essentially blaming them on the former Florida governor's brother, then-President George W. Bush.

This is an incontrovertible fact. Whether the attacks were Bush's "fault" is another question entirely. The attacks happened just nine months into Bush's presidency. And the 9/11 Commission report blamed the government's failure to stop the plot on structural intelligence problems that began well before Bush took office.

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Trump attempted a slight walk-back on Sunday, telling Fox News, "am I trying to blame him? I'm not blaming anybody, but the World Trade Center came down, so when he said we were safe, we were not safe."

But the caveat means nothing. By invoking the attacks the way he did, Trump is by default blaming Bush. And he took things even further by suggesting his tough immigration stance — which centers on building a luxurious wall between the U.S. and Mexico — would have stopped 9/11.

"I am extremely, extremely tough on people coming into this country," Trump said, adding that if he were president he doubted "those people would've been in the country. … There's a good chance that those people would not have been in the country."

For other candidates, claiming they would have stopped 9/11 would amount to an enormous, comical gaffe. But Trump makes a living on braggadocio and outlandish comments and so far the GOP electorate has eaten up.

Castigate Sen. John McCain's war record? No problem. Insinuate that Megyn Kelly was menstruating during a GOP debate? Higher poll numbers! Describe a plan for building a classy 2,000-mile wall on the Mexican border and somehow making Mexicans foot the $20 billion price tag? Sure!

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So there is no reason to think that Trump's 9/11 tirade — which of course extended to an ongoing Twitter fight with Bush — will hurt him. Figuring out when Trump has "gone too far" is basically impossible. We will only know it after it registers in the polls.

And at the moment, Trump still leads the GOP field, though he is now nearly tied with retired neurosurgeon Ben Carson in many national surveys.

This fact raises the question of why Trump is going after Bush rather than Carson. The short answer to this is: Who knows? It's probably because Bush has gone after Trump — in interviews and attack ads — and Trump really, really doesn't like criticism.

But it could also be that Trump thinks, probably correctly, that Bush is a bigger long-term threat to him than Carson. The more Trump makes the campaign about Bush's brother, the tougher it will be for the former Florida governor.

One other person quietly enjoying the current sideshow: Marco Rubio, who has inched into third behind Trump and Carson and ahead of Bush. At some point Trump will probably train his fire on Rubio. But for now the young Florida senator can sit back and watch the circus.