AHC/Plausibility Check: Sienese Empire

Here's your challenge friends, if you would humor me.
I'm hoping to write a timeline following the development of a sizeable and powerful (though non-wankish) Empire emerging from the Republic of Siena in Tuscany. There are a few points that I was looking at exploiting, particularly the Battle of Marciano in the Italian War of 1551-1559, where the combined forces of France and Siena clashed with those of those of the Duchy of Florence, Spain, and Holy Roman Empire. Of note was the Franco-Sienese Condottiero General Piero Strozzi, who is credited with the development of the dragoon/mounted gunner infantry concept. Here's what I'm thinking of a POD:

28 June, 1554: The Senese (Italian for Sienese) troops under Leone Strozzi, the naval commander of the Franco-Senese effort, successfully repel the Imperial/Florentine assault on Scarlino. (In OTL, Leone is shot to death during the siege).

Dates not Yet Finalized: Piero, still supplied by his brother's fleet as opposed to OTL, fares much better in conquering Florentine territories and repelling assaults on Senese territory. His rival, Marquess Gian Giacomo Medici, abandons the siege of Siena to engage his in open combat.

3 August, 1554: The Battle of Marciano is a decisive victory for France and Siena, their lack of numbers being compensated for by an expert use of cannon. The Imperial Forces are routed, and "Il Medighino", as the Marquess was known, is captured.

After Marciano, the favor in Italy shifts drastically in the favor of France and Siena. Although the Imperials shift resources in that direction, the brothers Strozzi prove too formidable for them. The redeployment of troops from northern europe allows France to see more gains in Flanders and against Spain. Siena besieges Florence, and eventually conquers it, going on to seize Modena alongside their French allies as well. In 1562, when the war ends with the Peace of Ferrara, France gains direct control of those Savoian and Milanese lands which it did not already control, and Naples is divided between France and Spain. Amidst this, Siena is allowed to retain the conquered territories of Florence, Lucca, and Modena, founding the Invittissima Repubblica di Siena (The Most Noble Republic of Siena) covering nearly all of the territory of the medieval Marquisate of Tuscany. In thanks for their services, the Strozzi brothers Piero and Leone are appointed Pretore (Commander of the Armies/Pretor) and Amiral (Admiral) of the Republic, respectively.

In the future, I see the Republic as modeling itself after the Roman Empire, taking great pride in its mythical heritage as the legacy of Romulus' brother Remus. The Strozzi will likely become the hereditary protectors of the republic, and hopefully the empire.

What do you guys think? I'd like to hear your thoughts! Thanks and God Bless!
 
Lets see. Minor Republic trebles in size in a single war and is clearly a French client. Expect revolts among nobles, Florentines, Modenines and Luccines, and with France having grown this much expect a coalition of Spain, the Empire, the Pope, Venice, Ferrara, Mantua, England and any nobles in Savoy, Milan, Naples, Florence, Modena and Lucca who want to rebel to declare war on France and Siena. The Strozzi will most likely be excomunicated, and the division of Naples is unlikely to fare any better than it did OTL.

This just seems too much in too short a space of time. Siena could probably gain the areas of Pisa and Arezzo from Florence, effectively reducing that republic to the area around the city of Florence itself (and making it a nice little annexation for later), but any more than that is just going to cause a war against Siena. France will also need to gain less territory (perhaps just Nice and the bit of Savoy gained in 1860 OTL) in order to prevent a serious war from breaking out which could lead to Siena being stripped of her gains.
 
Lets see. Minor Republic trebles in size in a single war and is clearly a French client. Expect revolts among nobles, Florentines, Modenines and Luccines, and with France having grown this much expect a coalition of Spain, the Empire, the Pope, Venice, Ferrara, Mantua, England and any nobles in Savoy, Milan, Naples, Florence, Modena and Lucca who want to rebel to declare war on France and Siena. The Strozzi will most likely be excomunicated, and the division of Naples is unlikely to fare any better than it did OTL.

This just seems too much in too short a space of time. Siena could probably gain the areas of Pisa and Arezzo from Florence, effectively reducing that republic to the area around the city of Florence itself (and making it a nice little annexation for later), but any more than that is just going to cause a war against Siena. France will also need to gain less territory (perhaps just Nice and the bit of Savoy gained in 1860 OTL) in order to prevent a serious war from breaking out which could lead to Siena being stripped of her gains.

Just the kind of point that I was looking for. The outline was hypothetical, to use as a starting point. So, with Pisa's port and Arrezzo's hinterland in their hands, and a (likely) interdict or excommunication when Papal authority was less than stellar, France could take some of Savoy and Milan. What could you see as the next step?

Ideas?

PS: Thanks for the thoughtful reply. This has been exactly what I've been looking for from this thread.
 
I have to agree that the terms of the piece treaty are too exaggerated: France is gaining too much, and Siena's reward are absolutely ASBish. Even your revised terms (Arezzo and Pisa) are too much: Arezzo and a substantial indemnity would be more than ok. The other problem I can see is that ITTL Siene would be no more than a French client.

Just a minor nitpick: "Invittissima" translates as "never defied"; "most noble" would be "nobilissima" in Italian.
 
I know very little of the dynamics of Italian politics and French intervention there during this period; the general impression one gets is that it is a big mess.

Basically I wonder, why should France aid one particular city-state, be it Republic, duchy, or whatever, rather than play a divide-and-rule keeping a whole cluster of minor allies on its leash, with the potential of taking a wayward one down by favoring the interests of a less fractious one?

Thus if Sienna is eventually going to establish itself as a strong regional power, but begins as a French client, there are two hurdles to overcome; one is to gain power despite French interest in limiting the ambitions of each of its stable of clients, and another would be to overcome the stigma of having been a dependent lackey of France.

One way to gain within the French system is to prove particularly loyal and helpful to French ambitions and thus both widen the field of French-controlled spoils to be awarded, and to always appear as both stern agent of French interests when some other client gets out of line and thus worthy recipient of territory and concessions stripped from these rebels.

For one thing that would be a limited path to success since surely the French will get suspicious of Sienna at some point, for another it makes the second hurdle all the higher. Not to mention that the French will hardly prove to be invincible in the long run!
 
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