The Republican presidential candidates are locked in a battle, started (naturally) by Donald Trump, over whether or not the 9/11 terrorist attacks happened on George W. Bush's watch. They did, of course, but being a Republican in good standing requires participants in the discussion to come up with extenuating circumstances about why it was really A Democrat's Fault.
Marco Rubio's entry goes thusly: Democratic President Bill Clinton did not kill Osama bin Laden years in advance of the attacks ever happening, so it's his fault.
"Well, I believe that if Osama bin Laden had been killed, Al Qaeda as an organization would not have grown to the point where it could have conducted 9/11," Rubio said. "And my argument was, no, the responsibility of 9/11 falls on the fact that Al Qaeda was allowed to grow and prosper and the decision was not made to take out the leader when the chance existed to do so."
For the record, this is something close to the “official” Republican position, to the extent that there is one: (1) It's Bill Clinton's fault for not killing bin Laden. (2) George W. Bush should not be criticized for not killing bin Laden. (3) It doesn't matter that Barack Obama did kill bin Laden so shut up.
[H]e made a decision not to take out its leader, which I think ended up being there, the situation that happened with 9/11. And as this was a response to an attack, that the reason why 9/11 happened was because of George W. Bush," Rubio said. "And my argument is, if you're going to ascribe blame, don't blame George W. Bush, blame a decision that was made years earlier, not to take out bin Laden when the opportunity presented itself."
This is all very complex. When it comes to assigning blame for things, Bill Clinton's presidential term extended a year past the time Bill Clinton left office because Reasons. When it comes to nominating Supreme Court justices, however, Barack Obama's presidential term ended a year before he left office because Other Reasons.
This is Marco Rubio is considered such a policy wonk, you see. Because of his ability to hold All The Viewpoints.