Earlier quoted context omitted.
War between who and who?
Not necessarily "large scale", but it's hard to leave the EU customs union without introducing a hard border between Northern Ireland and the Republic of Ireland, and specifically not having a hard border between Northern Ireland and the Republic of Ireland was part of the peace agreement that ended the Troubles.
Things would have been much less messy if Brexit had been implemented as a split of the EU, with the UK and Republic of Ireland running their own little 2-member EU government. Scotland could then be allowed to do their split from the UK while remaining in the mini-EU.