This is almost funny. One moment he's incompetent and useless, now he's superman achieving the impossible!My fear is that he can do a great deal of damage. Damage equal in magnitude to Brexit.
Seriously, his first action would be to agree a single market customs union with the EU, exactly what they want too. Then getting on with the trade deals which would be expedited by that new circumstance making our trading with the EU countries simple and easy.
As part of that he'd be trying to get the EU to agree to his desire to nationalise the railways and water, his primary aims. By then with at least two years passed, possibly much more since trade deals take time, how much harm is that?
I'm afraid that like so many, you've let hysteria take hold. Apply a sense of perspective and you'll understand that Jeremy Corbyn is no superman and will be very limited in what he can really change in one term, especially with so many Blairite MPs undermining his efforts and his balance of power very low. Remember, every step will need parliamentary approval for which he'd need all his MPs voting with him.
Reality is he'd possibly achieve nothing!
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