Well, Da Koach, consider the following scenario-which brings together my ideas for protectorates, culture flow AND an improved intelligence system:
You have three civs-Spain, France and Germany-all sharing the same land-mass. Around the early Middle Ages, Germany decides to make its move and launches a massive invasion into Northern France-with its Teutonic knights, Halbardiers and men-at-arms. The French meet the German offensive in several major battles, but is soundly thrashed-leaving just a few left-over defenders, some rapidly assembled militia and a devastated civilian economy! With German knights poised to capture the Capital-an act which could plunge the French nation into civil war and/or a Dark Age-Germany approaches the French leader and demands that he pledge alleigance to the German nation. Faced with little choice, the French player capitulates-handing virtual control of his nation to the German conquerers.
For much of the rest of the Middle Ages, France remains entirely under the German Yoke. Though they are not overly oppresive, they do tax the French nation heavily, and have large numbers of troops garrisoned throughout France. Cities in Southern France, in particular, are unhappy with German rule though, because they are all underdeveloped compared to their northern neighbours, and are beyond the reach of much of the German 'cultural juggernaut'-thus retaining a much stronger French identity than their northern cousins.
Meanwhile, Spain has grown fearful of the presence of a nation, close to its own borders, that owes its alliegance to their 'on again-off again' enemies, the Germans. Unfortunately, the Spanish lack the military needed to meet the German/French threat head on.
What they DO have, though, is an excellent Intelligence network, which they have invested heavily into investigation of both France and Germany (with some crossover due to the vassalage agreement). With the information that Spanish intelligence has gained, the Spanish player starts moving the bulk of its Intelligence 'machinery' out of Espionage, and in to Sabotage and Counterintelligence. Over the next few turns, Germany and France both start suffering from numerous acts of vandalism and destruction, with evidence implicating either the French or the Germans-depending on who the victim was. This creates major difficulties, as Germany is forced to move more and more of its forces into French cities in order to supress the early stages of a non-existent insurgency-an act which leads to increased anger and resentment in the cities of Southern France.
Eventually, Spanish Intelligence inform the Spanish player that they have an opportunity to spark a civil war within France, and if they should pursue it. The player says yes and, in the following turn, a dozen cities in the South of France declare themselves a new nation-independant of Germany and its puppet French rulers. In the upheaval, the German units stationed in these cities are expelled violently (suffering major damage in the process). Fearful of losing northern France as well, Germany moves its units back closer to their own border-reinforcing their position within the cities of Northern France.
In an act of brazen opportunism, Spain now opens diplomatic channels with the virtually defenseless new Republic of Toulouse, asking if they would consider becoming a Spanish Protectorate. Fearful of reconquest by the forces of Germany and France, and feeling a greater cultural affinity with Spain, Toulouse accepts Spains offer of protection-seeing a small sacrifice of independance as a small price to pay for such protection.
During the early industrial age, Spain uses its 'Toulousian Buffer' as a staging ground from which to launch an offenive against Germany's Vassal, France. In a desire to shore up its relations with Toulouse, Spain decides to 'Liberate' all captured French cities to their Toulousian friends.
Towards the end of the Industrial age, and with all of its strategic aims achieved, Spain dissolves its protectorate arrangement with Toulouse-granting the nation its full Independance-and gaining a staunch ally in the process

!
Yours,
Aussie_Lurker.