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.

Question: What led to his appointment?
Self-contained rewrite: What led to John O. Brennan's appointment as Obama's Deputy National Security Advisor for Homeland Security and Counterterrorism?