Defining War

I'd tend to think of "war" as being between two [or more] organised groups.* But if you have a character strong enough to take on a kingdom's military alone, then I guess the kingdom would consider itself "at war".
As for how it would play out, there would be so many variables.
What is the reason for the attack? What is the weapon? What are the armed forces of the kingdom? How organised are they? and a lot more.
* I'm ignoring things like the War on Terror/Litter/the Apostrophe etc.

 

Yeah, it probably can be. Even if it is one person. I've got at least one character like that, if she moves outside of certain set boundaries it's counted as an act of war upon the nation in question. If she comes with some of her knights, it's counted as a very aggressive war. But she can level even mountains with her fists, so even strong castles are something of only an annoyance. And at this point she has killed several armies too.

 

Well, it certainly could be but I think my first impression is that the kingdom getting decimated by one person with one weapon (assuming they must wield it in their hands and aren't just pushing a button) and assuming this is a world where there is a historical precedent for full on war as we know it, might not call it that themselves and try to spin it in some other way, using propaganda, to keep morale higher among their people and soldiers.

 

I would say it's not war, and the parties aren't likely to call it war. Perhaps the individual would say he's "at war" with the nation and give his actions that grandiosity that is ultimately self-aggrandizing. Basically, braggadocio writ large.

 

What does it matter?

Are you worried readers will raise an eyebrow and say: That's not a war!