The [Trump administration's] proposals don't call for constant monitoring once someone is in the country. It seems like [Saipov, the NYC attacker] became much more radical relatively recently. So the ideas on the table don't seem particularly relevant to this attack.
This is a movement that historically has been highly divided. One thing Osama had been doing is trying to be a unifier. He was very comfortable working with people who agreed with him on one issue and disagreed with him on five. Toward the end of his life, a lot of what he was trying to do was to get groups to work together.
What do you do when your allies [like Pakistan] are part of the problem? The desire to turn our backs on these people is there, but then you worry that terrorists will have more operational freedom and it will cost you more in the long run.
The [Barcelona] attacks, to me, show both the strengths and weaknesses. The strengths are obviously that [the Islamic State] has an array of supporters, especially in Europe, that it can call upon to do attacks. The weakness, though, is that it has had difficulty doing more sophisticated operations.