The deeper integration within the EU, and distancing from the UK on issues of major economic purpose such as currency, and a hundred other decisions like speed limits, trade rules and regulations, is what has complicated the withdrawal process.
None of which, or that im aware of, invoked a violent reaction.
If our absolute priority was zero prospect of a border on both sides, well neither the Irish nor UK government acted like that in their major decisions
Your talking about decisions taken at a time when there was a hard border. A conflict was ensuing.
And aside from the activities of paramilitaries, there was an unresolved political dispute between Ireland and Britain over the territory of NI.
The border was in place, and it required protection of British Army. That security border remained in place on the British side despite the advent of the SM, while customs posts were removed on Irish side.
there's no real reason why in 2019 it should mean violence unless people want a return to it.
Some people, a relatively small section of people, are prepared and preparing for violent attacks against security.
This is the often repeated assessment of the PSNI. It is evident in the form of attacks on PSNI resulting in deaths of PSNI officers, attempted murders and the death of Lyra McKee.
Its not a question of a return to violence, as it is a question of a return to escalating violence.
Why take the risk, when a simple NI only backstop can resolve the matter?
And most times the only way you'd notice that border line was those speed limits signs. Were any blown up?
Speed limits dont adversely interfere with the lives of people living in border communities.
Stop vehicles, looking for ID, searching cars, vans etc do.
It builds resentment, it puts people as targets.
And for the relatively small factions that are prepared to use violence it provides an opportunity to do just that.