Here is my thinking

Handed by Treaty (Notably, a Skyrim - led treaty, not an Imperial one) Skyrim ceded the island to Morrowind, this was done on the account on the devastating consequences of the Red year. Since then, the island was largely overrun by Dark elf people and culture, The islands ebony (The main economic resource) was bled dry by Dark elven miners. The island retain a strong undercurrent of Nordic culture, such as Skall village and Thirsk meadhall.

Presently, backed by the Empire, Skyrim has maintained cordial relations with its old nemesis, morrowind. Interestingly, the island has not been a political flashpoint between the two nations.

But, what if Skyrim suddenly becomes independent, backed by a pro - nationalist government, with a Nordic leader, relations between Morrowind and Skyrim would soon deteriorate. One day, Ulfric Stormclaak launches a sweeping verbal assult on Morrowind, claiming Solstheim as *the ancient homeland of the Nordic people*. With accuracy he describes the numerous battles fought by nords there, and how up until Imperial expansion of the island it had undisputable remained in Skyrims control. 195 years after the Red year, and with little economical advantage, Ulfric demands the island back (After all, he has likely expelled most non - nords from Skyrim now ((Remember Dagoth Ur's plans?)) )

However, Morrowind refuses, and the two countries slide to war.

Here are my Two questions

1. How likely do you think it would be that Ulfric would invade or even demand solstheim back?

2. How do you think the war would pan out? Who would win and what would the greater consequences be?

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1. In my opinion, this would be a likely consequence of an independent Skyrim. Ulfric would want to show that he is "calling the shots" now - and demanding back an island would be a great way to bolster support and nationalism in Skyrim. In likelihood, he would probably enjoy antagonising the Empire by declaring war on Morrowind.

2. In truth, this war could be the spark that lights the next great war. Skyrim would immediately launch a blistering Naval attack on the island (Its my guess, with a devastates coastline, Morrowind probably has a pretty weak navy, whilst evidence suggests Skyrim has a strong one). Nordic legions would quickly overrun Raven rock, destroying all elven culture and reclaiming it with Nordic settlers. Elven forces would retreat into the wilderness where they would rage a gurilla war on the Nordic occupation until greater Morrwind forces arrive. When they do arrive a long and bloody land war erupts. The Skall and probably thirsk would stay out of it. Depending on if they were attacked the Telvanni settlement would also remain neutral.

Eventually, the Eleven navy is crushed cutting off the military from fighting on the island, and so Skyrim inevitably reclaims it.

then...

Morrowind now appeals to the Emprie (Suddenly it reminds the empire how it is still a part of it) to send their own navy up the niben to help them. The Empire agrees, this results in a furious Ulfric launching a mainland invasion of Cyrodill. This conflict spreads quickly to Morrowind fighting across its borders. Interstingly, this could see Morrowind/the Empire becoming closer allies IF they defeat Skyrim.

OR

In response to the Invasion, Morrowind immediately launches a swift and bloody attack on the mainland, burning Riften and besieging Windhelm in a swift invasion to force Ulfric to relent. With line over-extended Black marsh invades from the south, fearful of over - extending its own lines the empire send no troops to deflect the invasion. Out - moved Morrowind is swiftly defeated, resulting in its territory shrinking even further. So that it is little more than a provincial Kingdom. Skyrim/ black marsh annex large swaths of Morrowind (Possible conflict building up there) and Empire, more fearful of Skyrims power, attempts to repair relations with it.

First off the Redoran Guard vs the Stormcloaks. the Nords would get their ass kicked. second of all i dont like ulfric but i dont think he would ever do that. he may not like the dark elves, but hes not stupid enough to go to war with them. his main enemies are the Empire and the Thalmor.

Ulfric will not attack anyone other than those two. This is a scenario that i dont think we will ever see.

Solstheim never belonged to Skyrim. Both Morrowind and Skyrim had claims on island, but Skyrim withdrawn it's claim.