I'd say Russia is still the top dog for potentially causing problems. They're currently playing the Nazi-Germany game of trying to take small places and hoping we'll go "eh not worth it" while they slowly creep into all of Europe.
Although I can more easily say what I believe WON'T trigger WWIII:
North Korea - I know at one point they had China's support, but I'm pretty sure even China wouldn't want to go to war with the US if NK gets uppity and decides to nuke something.
ISIS: May refuel or cause a war, but doesn't have the support of other countries to cause a world war.
I'd say Russia is still the top dog for potentially causing problems. They're currently playing the Nazi-Germany game of trying to take small places and hoping we'll go "eh not worth it" while they slowly creep into all of Europe.
Although I can more easily say what I believe WON'T trigger WWIII: North Korea - I know at one point they had China's support, but I'm pretty sure even China wouldn't want to go to war with the US if NK gets uppity and decides to nuke something. ISIS: May refuel or cause a war, but doesn't have the support of other countries to cause a world war.