Brexit creates four big strategic problems for the UK. In order of importance they are:

1. The likely end of the UK
2. The loss of national security
3. The loss of future energy security
4. Economic harm

And yes, economic harm really is only fourth.

A thread...

Brexit will likely end the UK.

This was widely expected of brexit before the vote in 2016, since Scotland exposed to Westminster (English) government without EU protection was likely to want independence. Polling now shows a big shift towards independence as a result.

England might hope to deny Scotland independence by simply refusing it. Global history however, shows that is highly unlikely to work. Having alienated our NATO allies with brexit, the UK will have little backing to deny democracy in Scotland if it wants independence.

Brexit in and of itself need not have hurt national security so quickly but it led to the rise to of Russian-linked people like Cummings and Johnson.

This creates security service mistrust among NATO and five eyes allies (especially when the USA ditches its Putin-pal). Also...

Brexit leads to likely Scottish Independence. That puts at risk UK strategic control of North Sea access - a vital security control.

It also removes the UK's nuclear naval base at Faslane, Scotland.

Both are major strategic assets within NATO.

Brexit hurts UK energy security

The UK historically had energy security, giving it great freedom from foreign influence.

Brexit diminished our prospects for resuming that in our post North Sea gas era - by diminishing our prospects for new single market supergrid connections.

Energy is a growing global challenge with giants like China increasingly active in securing foreign supplies. But the UK had an ace in the hole - off-shore wind and tidal power.

Losing Scotland due to brexit means losing a huge part of our land and energy-rich seas.

Brexit hurts the UK economy.

UK exports have fallen from over $800bn in 2015 to under $500bn in 2019 as uncertainty and plans to cut access to our export markets dramatically cut business investment.

The ways in which the econony suffers from brexit have been reported endlessly. Border delays, red tape, lost trade access and more.

But compared to the UK not existing anymore, diminishing national security and losing energy security, it's only 4th on the list of damage.