Mahatma Gandhi in WW1

In 1914, Mahatma Gandhi considered himself to be a patriotic British Subject, one who would form
the Indian Volunteer Ambulance Corps that year in support of those fighting on the Western Front.
As late as the summer of 1918, Gandhi supported the largest recruitment of Indian soldiers of the
war, to fight on behalf of the Empire. Yet by the summer of the following year he had become an
implacable foe of the Raj, aiming to undermine its endeavours at every turn. Why was that?