Sorry, for the massive quote, but I didn't want you to think I was cherry picking. The Stormcloak uprising is an excellent point relating to that which I was trying to make. Nation states don't use subterfuge when direct conflict is an option, as full war is usually faster and more effective at achieving those ends.
The Franco Prussian War is a good example of this. The Congress of Vienna that ended the Napoleonic Wars set a balance of power that heavily favored France in continental affairs. This was a long-standing thorn in the side of Prussia, which sought German unification through largely political ends of various sorts. So, when France thwarted a candidacy for a Prussian King of Spain, Otto von Bismarck finally was all screw this, we're stronger, let's just have it out.
And that's just the point. States typically adhere to large-scale subterfuge so long as the balance of arms remain outside of their favor. One that balance changes, direct conflict is the preferred approach.
Direct conflict isn't always the best option, while it may work or may not. The Thalmor seek to take all through conquest, but they plan centuries ahead. Now their tactics since they have gained power is never show of force, but divide and conquer. The benefit the Thalmor have is they rule with an iron first, they do oppress and force the provinces under them. The Empire doesn't oppress and it's provinces have greater freedom on things, now to the Thalmor using this subterfuge while not as fast, makes it easier in the long view.
Try thinking in hundreds of years. You want to rule all over Tamriel, now you have the option of show of force against a united group. The Legions can fight hard, the Dominion had made the mistakes of underestimating the Empire, just like the Empire underestimated the Thalmor. Now your goal is to rule over the lands with an iron fist, to do so you will need to maintain a large enough force to keep everything in order, so making your enemy fight themselves and divide them is exactly what the Thalmor want.
To them it's like a game of chess, you can either go straight out attack, wait for them to fail or you can make them create their own mistake. Now if they created hatred in one generations life time, that could easily be repaired if they went all out attack. If they create hatred in generation after generation after generation, they will force a rift between the provinces of the Empire that will be near impossible to repair. Cyrodiil and Skyrim have maintained generally close relations for Four Eras, Cyrodiil being the heartland of the Empire, Skyrim being the hammer of the Empire. Together united, Cyrodiil and Skyrim pose the greatest threat to Elven rule. Those two provinces conquered Tamriel together. They fought the Elves together in the First Era and again against the Aldmeri Dominion during the Tiber Wars.
If you cause a rift between Cyrodiil and Skyrim, the Empire will be greatly harmed. Nords hold onto grudges like a Swiss Banker, they hate for a very long time, after Four Eras, they still hate the elves.
The problem with show of force again, is they risk uniting Tamriel against them. The Thalmor aren't well liked within their own provinces. Many would jump at the occasion to over throw them given the chance. Now if they attack in force once more, the risk of uniting all the provinces will scale very high. Their attack united the Crowns and Forebears in Hammerfell, that was a feat.