The small sizes of armies in the Boshin War really ought not to surprise us in light of the political and military structures of the late Edo period. Simply put, Japan and its domains had not been on a war footing for a very long time, and were still in the process of developing their ability to mobilise manpower and materiel when regional conflict boiled over into civil war in January 1868.

Japan had by no means been totally at peace during the ascendancy of the Edo bakufu, but most domains and even the Shogun's tenryo had maintained very small standing armies. Moreover, only members of the samurai class (which at this stage included the ashigaru) were formally permitted to bear arms. When, in 1862, a delegation of mostly southwestern samurai visited Shanghai, they were surprised to find that the Qing (at this time still locked in an existential conflict with the Taiping Heavenly Kingdom) relied so heavily on peasant militias in its armies. This may well have served as inspiration for one of its members, Takasugi Shinsaku, when he established the Kiheitai in 1863 and explicitly disregarded social class in its recruitment. The Kiheitai were, however, only one part of a wider movement in Japan to widen the net for military mobilisation. Peasants would be enrolled into the Shogun's forces from about 1864 onward, at a rate of 1 man per 1000 koku of assessed grain output. However, given that the bakufu's land value was around 4 million koku, had this policy been sustained then this quota would have produced a mere 4000 peasant conscripts. Expansion, in the case of the bakufu, would be slow and conservative.

The more significant bottleneck, though, was access to weapons and training. Virtually all the domains of Japan at this stage recognised that the only troops worth their salt on the modern battlefield were equipped with modern rifled arms and trained in using them. Those with cold arms could, at most, help swell the ranks. However, such weaponry and expertise could only – in the short to medium term – be found abroad. Rifles would have to be purchased as there was not yet the ability to manufacture them domestically, and instructors would have to come from foreign powers. This was compounded in the case of the bakufu by its limited ability to actually extract its theoretically considerable wealth, as it took time to fund the purchases for the weaponry that it would need to equip a modernised army. It was also hampered in its efforts to solicit foreign training, as British diplomats rebuffed or ignored bakufu requests for support, which led to a gradual pivot towards France that would only really fully manifest in outright military aid beginning in 1867, right on the eve of civil war. For their part, British officers instead began providing their services to the principal anti-bakufu domains, Chōshū and Satsuma.

If we look at these domains, we find that their military power – helped considerably, no doubt, by British support – was actually quite substantial when compared to the bakufu's. In November 1865, the bakufu army at Ōsaka (including its supporting elements) numbered around 8000, of whom 6000 were properly modernised. By contrast, Chōshū alone was estimated by the bakufu's sources to be able to raise over 14,000 modernised troops. Granted, these numbers exaggerate the size of the anti-Tokugawa armies on the one hand, and on the other they do not account for domains allied to the Tokugawa clan which raised their own forces. However, these small and scattered entities were unable to provide many modernised troops, and in any event this reliance was quite damning given the ostensibly far greater wealth held by the Shōgun when compared to the rebelling domains of the southwest. Even by the outbreak of war 1868 this number had not increased that considerably (although defeat to Chōshu in 1866 did mean the army had to be reconstituted), with perhaps 7000 modernised bakufu troops – augmented by local auxiliaries and the forces of allied domains – marching northwards to Kyōto to fight at the Battle of Toba-Fushimi. Granted, these troops were the result of further reforms after 1866, but the bakufu was still far from being able to effectively attempt a mass mobilisation for war. By contrast, Chōshū's early eagerness to recruit peasants and merchants, and its acquisition of foreign arms and expertise thanks to Britain's faux-neutrality, meant that it and its principal ally, Satsuma, internally assessed that they could assemble at least 20,000 modernised troops between them, along with any forces mobilised by their allies.

Conditions in Japan ca. 1868 were really quite different from those of the Napoleonic Wars or the American Civil War. By the time of the Napoleonic Wars, European states had been on a war footing for centuries and had developed considerable mobilisation systems as well as domestic arms industries. While the French Revolution would see these expand even further, this was built on quite considerable foundations. Impressive as the alleged 1.4 million Frenchmen under arms in 1794 was (though actual strength likely did not exceed 800,000), this was not even an order of magnitude increase over the perhaps 250,000 troops – nearly 400,000 on paper – that the Bourbons could field during the War of the Spanish Succession less than a century earlier. The United and Confederate States may have split a tiny standing army between them, but both had large militias, some level of domestic arms production, and – at least to begin with in the Confederacy's case – the financial power to purchase weapons from overseas, allowing the two to create high-intensity war efforts off the bat, and for the US to sustain its for the better part of half a decade. The Japanese domains simply did not have the same baseline around which to coordinate a rapid military expansion.