Hm, well, the US are describing this as a "war on terrorism".
We learned in the past and now too that's it's incredible hard to wage war on an adjective. I'm not entirely sure that the word war is appropriate.
War \War\, n. [OE. & AS. werre; akin to OHG. werra scandal, quarrel, sedition, werran to confound, mix, D. warren, G. wirren, verwirren, to embroil, confound, disturb, and perhaps to E. worse; cf. OF. werre war, F. querre, of Teutonic origin. Cf. Guerrilla, Warrior.]
1. A contest between nations or states, carried on by force, whether for defence, for revenging insults and redressing wrongs, for the extension of commerce, for the acquisition of territory, for obtaining and establishing the superiority and dominion of one over the other, or for any other purpose; armed conflict of sovereign powers; declared and open hostilities.
Now there are three passages here that are interesting:
redressing wrongdoings,
or for any other purpose and
declared and open hostilitiesBut these apply to state vs state issues.
One could argue that we now have a war on terrorism, since we're not going to allow it and will actively go after it. Then again, the same could be argued about the breaking of laws, or to be more specific, war on drug use, war on breaking and entering, war on...ad nauseum.
With the label of "war" comes a great deal of things. First of all, war is incredibly ugly and messy. Deplorable things will happen; in war and love anything goes. It's a Carte Blanche for the use of (excessive) force, for ignoring parts of what under a peace time situation would be important parts of a countrys philosophy.
WWII was pretty straightforward; Germany declared war on the USA and engaged in a war much like the definition above. The Japanese preferred a stealth attack first, but it was a regular war. This one is not a regular war at all. It's more of a huge policing effort with lots of use of military equipment and methodology.
I know the US would like a Carte Blanche check from its US allies - and it has got it from most if not all of the NATO allies. Yet, I feel it is *essential* for each country to be able to individually assess how they want to address the military aspect of the situation. 100% solidarity and support does *not* mean giving up sovreignity or decision making. In addition to this, lots of European countries have constitutions that have to be taken into account - we do not operate exactly like the US. That must be kept in mind when dealing with this.
Last of all, there are elements in society that don't *want* military strikes. And we're democracies - no matter how much distaste I have for people with this opinion, I still have to respect their opinion or vote.
So, Europeans are doing as best they can, but we're internally divided to a much larger extent than the US is. I mean, toejam, we still got communists etc *winning votes* in our parliaments. Sickening but true, unfortunately

So far, Denmark hasn't ruled out lending military support if needed. Of course, we don't have a helluva lot, but what we DO have is a rather large-ish immigrant population and according tosome experts, Scandinavia is a settling ground for terrorists since civil liberties is taken extremely seriously, to such an extent that they aren't terribly bothered by our security agencies.
This will change I am sure. As we speak, or minister of justice is meeting with his European counterparts to find ways of fixing it.
What I'm saying is that there's a lotta factors involved, and the usual "you're either with us or against us" attitude, while understandable, isn't applicable to the real life situation. It's far more complex than that.