there will probably never be anything defined as a world war anymore

I wish I was as sanguine as you are, Jazzo, about the possibility of mass exsanguination. Experience suggests to me that when we all lean towards one belief, the opposite tends to occur - never be surprised by the frequency of surprise!

My answer to the original question would be that World War is defined as war involving countries on every continent.

And tho' only an amateur historian, I would say the effect of codebreaking in WW2 was massive - Enigma was probably the major contribution to keeping supply lines open for the UK and therefore the European mainland - yet still not decisive. For example, US troops still took massive casualties in the Battle of the Bulge due to complete absence of prior warning of the German battle plans - so in this and other key episodes, what counted in determining the final outcome was the strength of purpose of the man prepared to die in the mud. Perhaps no intelligence or codebreaking system in the world can ever equal that effect, as the CIA are wondering to their cost.