FWIW, I posted an historical accounting of this a while back essentially summarizing Bodiford. The refs and URL follow:
For a very enlightening read--revisionist even--see Bodiford on Religion and Spirituality in the Martial Arts: Japan in the Martial Arts of the World Encyclopedia.
Title Martial arts of the world : an encyclopedia / edited by Thomas A. Green.
Call Number R 796.803 MA
Publisher Santa Barbara, Calif. : ABC-CLIO, c2001.
In the teens, 20's and 30's, Jp was in the clutches of fascists who oriented education to inculcating sentiments of suicidal allegiance to the emperor. The Jpn recognized their technological incapacities and intended to take up the slack with "SEISHIN", fighting spirit (remember the women in Okinawa fighting flame throwers with sharpened bamboo poles? One modernizer who tried to build up Jpn armaments was accused of treason).
To this end, the famous Hagakure ("The way of the samurai lies in death"), written by a romantic gasbag born during the peace of Tokugawa who never had to draw his sword in anger, was widely circulated to inspire fanaticism; martial arts were taken over by an organization called the Butokukai founded for this purpose to introduce youth to fighting and sacrifice; Momotaro, a children's story about a superhuman toddler who drives off the long-nosed barbarians, becomes canonical.
Samurai had become unwelcome in Meiji (1868-1911). They were conservative dinosaurs in a time of cataclysmic change. Nitobe Inazo, a Quaker (I think) wrote Bushido, in English, to reconcile Jpn values with Christianity. After the Jpn womped the Russians, however, an event inspiring peoples throughout the colonial world where whites had theretofore been regarded as undefeatable, values of the samurai were reconsidered. "Bushido" (Nitobe had thought he invented the term which had alternately been referred to as "budo", "samuraido", etc.) was appropriated by the politicos and "DO" took on the meaning of emperor worship (Here, Bodiford explicitly corrects Draeger who denies this history).
A police superintendent wrote that "bujutsu" ought to be written "budo" and this soon occured. In the 30's, the term "dojo" became widespread; borrowed from Buddhism, it lent a patina of spirituality to the rough business of preparing an army of suicidal maniacs. Constabularies regularly policed dojo to enforce the requirement that they have KAMIDANA at the front of their practice area, and bowed to it before and after class.
Offers new perspective to the standard "harmony of the universe, self-perfection thing", doesn't it? Kano, founder of judo, must have rolled over in his grave and it's said that Ueshiba Morihei retired to the countryside to avoid being part of the prostitution of his art thus.
Evidently, after the war, many martial artists acquiesed to the association of their arts with Zen through what had become "The Ways", not because it was actually so, but in order to rehabilitate their practice with the appearance of social utility. YMMV.