War.

What justifies it?

We went to one place because we suspected they had weapons of mass destruction and felt justified doing so.

Now, another (who I might add isn't particularly friendly with us either) is straight out telling us they have and will soon be testing nuclear weapons is just being told "Stop it! That's bad!"

Hidden motives perhaps finally coming to light? Or just one country is more of a threat than another?

There's something to be said for consistancy in foreign policy.