By far the biggest worry is that in the chaos engulfing Syria, some chemical weapons might fall into terrorist hands.

ECONOMIST: Syria and its chemical weapons

The UN's resolution 1540 obliges all governments to stop nuclear (and chemical and biological) bomb-making materials falling into terrorist hands.

ECONOMIST: Preventing nuclear trafficking is easier than policing it

There are risks that an Iranian nuclear weapon could fall into the hands of a terrorist organization.

WHITEHOUSE: President Obama at 2012 AIPAC Policy Conference

And then we return here to focus, as I mentioned, with 46 other countries, on locking down any vulnerable nuclear material over the course of the next four years to ensure that we are not faced with that kind of material falling into the hands of a terrorist organization.

WHITEHOUSE: Press Briefings

Fatah leadership has had a great deal of support from the West and has been warned over the years not to join hands with militant Hamas, which several countries have deemed a terrorist organization.

CNN: Gaza rally called step toward unity for Fatah and Hamas