So states can only be in a state of war with other states? Well you learn something new every day I suppose. So the British government were never - legally - at war with the IRA?
Do all the ISIS declarations of war and attacks, and the fact that British troops have been involved in fighting them over in Syria and Iraq, count for nothing then when we’re weighing up the legalities here?
I may be slightly wrong I'm not an expert in international law, but I believe technically yes.
Which is to say that technically 'war' describes a state of relations between two countries. i.e. in Chamberlain's declaration against Germany:
"This morning the British Ambassador in Berlin handed the German Government a
final Note stating that, unless we heard from them by 11 o'clock that they were
prepared at once to withdraw their troops from Poland, a state of war would
exist between us.
I have to tell you now that no such undertaking has been received, and that
consequently this country is at war with Germany.
You can imagine what a bitter blow it is to me that all my long struggle to win
peace has failed. Yet I cannot believe that there is anything more or anything
different that I could have done and that would have been more successful...'
If you don't recognise the other party as a sovereign state, you can't be in a state of war with them and you can't, therefore, be at war with non state actors. It's a bit of a relic from the nineteenth early twentieth century though because if we're being really strict with it then neither the UK nor the US ever went to war with Iraq.