Such a move would kill the NATO alliance, which is another important political consideration.
But I don't think the European partners are able to intervene in North America in any meaningful way. US air and naval forces are too strong for them to cross the Atlantic.

What could happen in an extreme ...

But how to keep this challenging?
The US army is much stronger than the Canadian one, same for mobilization potential. And as the US also dominates the Atlantic, so there is little hope of reenforcements.
The main reason for the US not to do it are political and economic. Canada is hard to absorb ...

Anyone has the original scenario?
The one in the scenario archive is not the right one.

Please tell me you make an Indonesian revolution or Malay emergency scenario

Yes, it is contemporary.
Basically the UK continues to deteriorate and the nationalist Scottish government begins pushing for independence.
Once the nationalist start building a Scottish National Guard as quasi army, the British government decides to remove the nationalists from power.

It is a ...

The British Army invades Scotland to remove a secessionist government, which is in the process of building its own army.

An attempt to translate a Balkan / Middle Eastern Style conflict to Western Europe.
Might have gotten carried away, when adding loads of colorful units and sub-factions. Hope it ...