Technically there was no peace treaty between ICSN and NPO back in the first few months of the game (ICSN was curbstomped. NPO was satisfied with the result and just walked away). If those kinds of things are carried on from alliance to alliance then...
ICSN became ICP became SWF became CPCN became INT
It's quite a stretch, but by the same logic INT is, and always has been, at war with NPO.
http://en.wikipedia.org/wiki/Berwick-upon-...ons_with_Russia