Why do we need the union?
Good question. I guess it comes down to personal philosophy. At some point humans have to group together,
- tiny (a family)
- small (a tribe or village)
- medium (city states or regions)
- large (nations)
- very large (federation of nations)
Of course some places blur the lines is the USA and the EU are roughly comparable but the USA is a tight federation of medium nations, whilst the EU is a loose federation of large nations, some of whom are federations of smaller regions (Italy, Germany).
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Essentially, everyone has to make a decision as to where they think the "sweet spot" is. Small has advantages in that an individual's voice can be heard, large has advantages with being able to do bigger things. I might have a much better chance of being heard at the parish council, but the parish council isn't going to be able to mount a mission to mars (not unless the allotments can put there)!
It was created at a time of insecurity in Europe when it made sense for there to be lo possible land borders to defend. Sea borders, at that time were easier to defend, especially with the then English, soon to be British navy.
But these issues are no longer valid.
Sadly, I think that the unstable times may be starting again.
Boarders of any kind are not very important.
As a snowboard instructor I find that offensive!

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I wish that were true. Unfortunately people like to live differently from each other. Whilst I am happy to live and let live, at some point people's personal preferences have to be transformed into public laws. For example should women have to wear a veil/headscarf? Personally, I don't care, if some one wants to wear a head scarf, a veil, a burka, a Guy Fawkes mask then fine. If they don't then also fine. However, some out there may insist that any head covering with a religious theme should be forbidden in public, whilst other think that wearing a veil/burka, headscarf should be mandatory for women in public. At some point we have to draw a geographical line and say "on this side, we have decided veil=compulsory and on your side veil=optional". This goes for all sorts of things, gun ownership, clipping of dogs ears, producing fois gras, topless bathing, gay marriage, drink driving limits, healthcare provision and all the little things that add up to differences between cultures and nations.
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In an ideal world I could live and associate with likeminded people and not have anything to do with those that we disagreed with. I may have more in common with someone from California or Kenya than I do with the person who lives next door to me (having met some of the "retired brigadier" types in the village, I can defiantly say we are not on the same page on a lot of matters - "the gays!", "those bloody foreign types", "bloody good thrashing is what they need")
So, why do we need a UK union?
I creates friction and jealously.
You will always get friction in any group, family holidays are a great example ("I don't want to see the lousy ruins, I want to go to the water park"). Our local village is in the middle of trying to get a planning document agreed, the arguments and accusations are a sight to behold, it's like game of thrones but with less sex and more biscuits.
It allows governments to divide us. Thatcher for example.
Whilst Thatcher was divisive, no government is ever going to be able to represent all of the people all of the time.
A government's job is to arbitrate between the competing requirements of it's citizens. No government will ever be able to please everyone all of the time. Governments will always be disliked by someone.
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Take thatcher, many would damn her for destroying the mining communities etc. Yet she was also instrumental in setting up the Intergovernmental Panel on Climate Change (IPCC) and was one of the first world leaders to start to tackle mankind's effect on the climate. If you are keen on preventing/mitigating climate change then Thatcher was good for your cause. If you liked digging up coal, not so much.
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It is clear the states of the UK are capable of governing themselves. So claims that we need the UK because we are not capable of governing ourselves without making a mess of it are a nonsense.
Yeah, Wales, Scotland, England and NI could all govern themselves, they are not children. But let's imagine what might happen if the UK vanished
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England would probably be ok, as long as we could keep a lid on our bloody banks!. The new English Pound (Stirling is dead) would probably drop a bit, times would be hard as we would have to readjust to our smaller stature. Getting the deficit debt under control would be tough. But would be stop at England? What about Cornish independence? Maybe, London decides it's fed up of producing 20% of the GDP and decides to push off on it's own as the Republic of London and the South East?
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OK, Scotland would march off and implement iScot, which, as we discussed, has a solid economy. it would have to face the economic headwinds of trying to reduce it's deficit and debt. it could devalue it's currency (as it would have to have it's own currency) to make it's industry more competitive. Yup, iScot would bumble along just fine.
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What would NI do? It's southern neighbour would like to absorb it, but what about the significant chunk of the NI population who don't want to be part of Ireland? It could stay as an independent state on the north of an island (just like scotland) but it's economy is not as strong as Scotland or Englands. It would be a functioning state, but what if the old troubles re-emerged? NI could slide the way of Yugoslavia/Ukraine/Nigeria as civil war breaks out. The Irish wouldn't be very happy with a civil war running in the northern tip of their island.
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The welsh economy and public finances are nowhere near as robust as Scotland's. I don't think that anyone seriously claims Wales would be better off financially if it was it's own state. So wales would have to tighten it's belt, cut services and up taxes. At least it can use it's own language at last, and have it's own sports teams and TV.....
It's been a fun ride, true, it's been a barrel of laughs. But all good parties come to an end. Pleading to nostalgia and Conservationism are kinda pointless
So, why do we need the UK.
To go back to the beginning, What is the optimum unit of civilization? I would argue that bigger is better, not just the UK but ultimately the EU and bigger. Of course the caveat is that that people should want to be part of the bigger unit. Forcing people to join is empire. The bigger unit's advantages should speak for themselves, ok we have to give up X,Y and Z, but we get A, B, C, D, E ,F and G!
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Maybe we should work up an independence package for each nation with exactly what happens spelt out (keep/not keep the pound, how much debt, what oil etc). Then every 5/10/20 (pick a number) years have a referendum on it. That way everyone can be sure they are in the union because the chose to be not because they are forced to be.#
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Finally, Given this is an Elite Dangerous forum, if we are ever to get of this rock and actually make ED (as in human colonisation of the stars, not as in murderous corporate dystopia!)a reality we will need the resources of EU/USA/USSR/China scale at least. We should be emphasising our similarities and shared goals for the future, not pointing out our differences and dragging up the past ("Well 300 years ago, your ancestors were a bunch of -holes to my ancestors, plus you speak funny, so we should go our separate ways")