"Refuting U.S. claims, the paper went on, “The administration has shown its embarrassment in other ways. On day two of the war, Donald Rumsfeld, the Secretary of Defense, said finding and destroying weapons of mass destruction was the invading force's number two priority after toppling Saddam Hussein - itself a reversal of the argument presented at the UN Security Council. “A week later, Victoria Clarke, the Pentagon spokeswoman, pushed the issue further down the list, behind capturing and evicting "terrorists sheltered in Iraq" and collecting intelligence on "terrorist networks". "Now we are told that hunting for weapons is something we can expect once the fighting is over, and that it might go on for months before yielding significant results. "It's hard work," a plaintive Ms Clarke said last week."

Ms Clarke thinks she has a clue, but she doesn't. US policy is consistent.
[[Oops, it's not Ms Clarke, but someone writing some stupid article- the punctuation misled me- but I stand by the rest.]]
After some fools flew a couple airlines into the World Trade Center, US policy shifted. The stated number one priority of the US national security is is the fight against Global Terrorism. PERIOD.
The fact that even NATO recognized the fact that the US was attacked and invoked it's treaty obligation, should have caused *some* to take notice.
Btw, in general terms our priority hasn't changed since our country formed: Life, Liberty, and the pursuit of Happiness.
Now, the main legal argument for toppling Saddam's regime was that fact that that country was violating a cease fire agreement [therefore was still at war with the US & and one of these violations was concerning WMD (which was a particular concern in terms of potential terrorist attacks)].
An additional Btw, since N Korea has not legally ended it's war with S Korea, legally the US *could* attack N Korea. Just because we can legally attack a country does not mean we will attack a country. But since our country respects the Rule of Law, it would wise if countries not wishing the possiblitity of the US attacking them to avoid doing things which legally allow the US to attack them. State sponsored terrorism is a act of war. By starting or continuing a war with the US, the US *might* go to war with you. By agreeing not to continue attacking the US and you wouldn't be attacked- as long as you don't actually continue attacking. If we have proof of a country committing act of war against the US, then we are going to want to have *some* verification that this activity has ceased.
Also another freaking obvious point [since I'm ranting], a sovereign power stating it's at war is an oral declaration of war. Legally there is not really much significant difference between writing something down and verbally stating it- especially if your voice is recorded.
Another pretty obvious point is that if the US is sending covert operations against another country this is also an act of war- and generally it's a response to that country sending covert operation against the US or one of it's allies. We don't need to conduct overt wars involving tank columns and air power, nor is there much difference between covert and overt war- other than the PR and covert wars are generally far less expensive.