Greatest Threat to the US?

According to Michelle Flournoy (likely the first US female SecDef), it's rogue WMDs, so we better get ready to secure the Norks nukes when they finally fall apart.

The risk of weapons of mass destruction falling into the hands of terrorists is the gravest threat facing the United States, a Pentagon official has said.
Asked about the existential threats against Washington, Undersecretary of Defense for Policy Michele Flournoy said there were many, but warned that "the thing that keeps me awake at night is a nexus between terrorism and massive destruction."
She told a Washington symposium on US security that the United States needs to better prepare for "the possibility that a terrorist organization could either acquire a ready-made weapon or fabricate something improvised that would have a catastrophic effect for us."
The threat of WMDs falling into terrorist hands was the driving rationale behind high-profile nuclear security summit hosted by President Barack Obama in April which featured leaders from 47 countries including "nuclear club" members China, France, India and Russia.

By: Brant