I don't know if Donald Trump will win the Republican nomination. But even if he doesn't, it's increasingly clear he's going to destroy Jeb Bush before he loses.

Over the past week, Trump and Bush have been in an argument that basically boils down to the question of was George W. Bush president on 9/11/2001?

Trump insists that Bush was president both prior to and during the 9/11 attacks, and he was therefore at least partly responsible for the security failures that permitted the tragedy. And to Trump's credit, there is considerable evidence that George W. Bush was president on 9/11/2001.

Jeb Bush's position is harder to parse: He argues that his brother was only responsible for what happened after 9/11, suggesting, perhaps, that someone else bore the responsibilities of the presidency on 9/11/2001. Or, to be a bit kinder to his position, he argues that the measure of responsibility as president isn't whether something like 9/11 happens, but whether it happens again.

How pathetic for @realdonaldtrump to criticize the president for 9/11. We were attacked & my brother kept us safe. — Jeb Bush (@JebBush) October 16, 2015

The result is this absolutely brutal interview CNN's Jake Tapper conducted with Bush. "If your brother and his administration bear no responsibility at all," Tapper asks, "how do you then make the jump that President Obama and Secretary Clinton are responsible for what happened at Benghazi?"

Bush's response is almost physically painful to watch.

Trump has a bully's instinct for finding someone else's true weaknesses. His continued crack that Bush is a "low-energy" candidate is devastating precisely because it identifies a weakness not just in Bush's campaign style, but in the nature of his campaign.

Now Trump has pulled Bush into an even more dangerous quagmire: his brother's presidency. Trump is reminding every Republican voter that nominating Jeb Bush will mean running a general election campaign with two disadvantages. First, Republicans will have to answer for George W. Bush's failures in a way they wouldn't if they nominated Marco Rubio or Carly Fiorina or Donald Trump; and second, they'll need to somehow explain why they're holding Hillary Clinton responsible for Obama's presidency even as they don't hold George W. Bush responsible for George W. Bush's presidency.

And Trump, having realized how weak Bush is on this issue, isn't stopping. He's moved from 9/11 to the Iraq War: