Conversation so far:
When did John O.
Brennan's time as a counterterrorism advisor to Obama begin?
President Obama then appointed him to be his Deputy National Security Advisor for Homeland Security and Counterterrorism, the President's.
What led to his appointment?
In August 2009, Brennan criticized some Bush-administration anti-terror policies.

Question: Did he have a hand in any historically significant counterterrorism?
Self-contained rewrite: Did Brennan have a hand in any historically significant counterterrorism?