Did you know you're at war with North Korea?

Did you know you're not at war, and have never been, with Afghanistan?

Meaning of all that: what is or isn't "war", an "act of war" or "is" is up to people to define, and international law is easily ignored whenever states think that's a good idea.

The best definition is probably the UN's "act of aggression", see http://www.un-documents.net/a29r3314.htm. That definition does not include provisions for such situations – the only (theoretically) unarmed act of aggression is a blockade.