King Abdullah (Saudi Arabia)

Why? Since 2003, Abdullah has led a crackdown on jihadists and maintained close ties to the United States. He is also modernizing the kingdom, a dangerous step in the deeply conservative country.

Who? Sunni extremists, either linked to al Qaeda or acting independently.

How? A suicide bomber or a member of the Saudi security force loyal to the jihadist cause.

Then what? The corrupt and inept crown prince, Sultan bin Abdul-Aziz, is the heir apparent. Although he is pro-American, Sultan is a bit of a dilettante, so his brother, the competent but harsh (and anti-American) Nayef bin Abdul-Aziz, might run the kingdom while Sultan frolics.

Worst case: Sultan takes power but is largely incompetent. While he fiddles, Nayef exercises de facto control, crushing any reform movement and distancing the country from the United States.