WI: The Shimonoseki Campaign escalates into all out war between Japan a alliance of Western powers

In 1863 and 1864 there was a series of naval battles between an alliance of France, Britain, America and the Netherlands against the Chōshū Domain, who were taking the "Expel the Barbarians" part of "Sonnō jōi" philosophy very seriously. In OTL the conflict remained restricted to the Shimonoseki Straits and was primarily naval, and in the end a diplomatic solution was reached because the central government in Edo did not support the Chōshū rebels.

But what if diplomatic negotiations broke down, and the Western Alliance declared war on Japan? Let's say their military objective is to arrest the Emperor and force him to rescind his "Expel the Barbarian" edict, and the Shogunate ends up being unable to met such a outrageous demand without losing all legitimacy.

How long would would pre-Meiji Japan, divided and backward as it still was, last in a war against the Western Allies? Apparently the West were quite shocked with how many modern weapons Chōshū had gotten their hands on.

America would probably only be able to provide token support, busy as they were with the Civil War at home. Perhaps a brief fad of Japanophilia occurs in the Confederate press, but probably little more. America will probably be at a disadvantage at the negotiation table when the conflict ends.

Upon defeat, could Japan be partitioned between France, Britain and maybe the Netherlands?
 
And France was busy with the Mexican intervention. Because of the other conflicts im not sure Brittain is willing to engage itself sufficiently to conquer Japan - keep a free hand in case it needs to intervene in one of the two conflicts mentioned. The Ntherlands is not that important at this point. So I think the most you will see is a bit more serious naval action but no full scale invasion of Japan.
 
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