The North Korean tests:
Rhetoric aside, the US and its allies have no realistic means with which to punish North Korea. Military action is widely considered unthinkable. This is not merely because the US is militarily stretched in Afghanistan and Iraq, nor is it because the North could retaliate using its nuclear weapons. It is also because Pyongyang has massive conventional weaponry, including a 1.1 million-strong army (the world's fifth-largest), 180,000-strong special forces (the world's largest such force), and thousands of artillery pieces and short- and medium-range missiles capable of raining destruction on South Korea and Japan.
... plus the Obama factor. The wrong appeaser at the wrong time.