Iceland benefited by the Nazi invasion of Denmark. With the King under house arrest, Iceland went ahead and declared independence. They joke that Hitler was their liberator. Of course, the presence of a US base helped them out too.
There was a contingent in Ireland that believed a Nazi invasion of Britain would free Ireland. That was a somewhat shortsighted assessment on their part.
In fact, Norway found itself unwillingly part of two unions that together lasted about 500 years. During the first 400-odd years, from 1397 through 1814, Norway was part of Denmark. (The first century of that actually involved the Kalmar Union, but let’s keep this simple.) From 1814 through 1905, Norway was in a union with Sweden.
I would argue that the union with Denmark on balance was bad; Norway was treated as the backwater provinces much of the time, and got dragged into every war Denmark fought - and they fought a few during those centuries - whether it served Norway’s interest or not.
In 1814, Denmark had to pay the price for backing Napoleon, and the Great European Powers decided that the appropriate punishment was to take Norway away from them. Then they decided Sweden should be rewarded for fighting against Napoleon, and the appropriate reward was to give Norway to them. As far as I can tell, nobody asked any Norwegians what they thought about the whole arrangement. What they thought about it, as a rule, was that it was a bad idea. A group of men quickly drew up a constitution, and two Scandinavian countries fired shots in anger at one another for the last time. (Mind you, the whole 1905 thing was a bit touchy, but no shots were fired.) The result was a compromise that gave Norway far more rights than it would have had under the original plan - far closer to a union of two countries than one country swallowing up the other.
On balance, that union probably benefitted Norway. As a temporary solution, it gave the country a chance to build national institutions, and rebuild a sense of national identity, while being protected from being dragged back into the union with Denmark. Over time, the solution became unworkable, which is why the two countries are separate today - but things could have gone far, far worse for Norway in 1814.
And DeValera refused to negotiate with Churchill on the use of Irish ports because he considered he was negotiating with the loser. Which was more than shortsighted.
Between the times of Dunkirk and the attack on Pearl Harbour it wasn’t really that shortsighted. The UK wasn’t in a very good place.
It also would have almost certainly led to another civil war. Montgomery thought that it would be a waste of time due to the amount of men that they would have had to put into Ireland to protect the ports from internal attack from the IRA. He was based in Cork during the War of Independence BTW so he knew the lay of the land in Ireland.
Nothing much to add, but here are some things the Danes did other than drag us into wars:
Take our silver, both out of the mountains and out of our churches (reformation happened during Danish rule).
Take our copper.
Take our timber.
Take our sailors.
Take our domains. Iceland, Greenland, the Faroe Islands, the Orkneys, and Shetland were all Norwegian before entering into the union with Denmark. The Danish king pawned the Orkneys and Shetland to James III of Scotland, and the rest stayed with Denmark when the union ended in 1814.
What did we get in return? Not much worth having.
A lot of European countries kept the Napoleonic Code for their legal systems, even after they got rid of Napoleon himself.
Gaul, Iberia, and Britannia were much better off in the long run for having been civilized by Rome.
When the Romans conquered Greece, they were apparently so impressed by Greek culture that they adopted much of it (including the Greek religion) entirely. To the point that 2000 years later we still refer to ‘Greco-Roman heritage’.
I’m not sure if that was much benefit to the Greeks, though. They were still a Roman colony. But they did get treated better than some of the ‘uncivilized’ lands, such as Brittany or Germania.