As Jack Censer’s post has pointed out, the role of the military in revolutionary situations is critical to understanding them. Yet, it varies so much that finding common threads can be extremely difficult, and even then misleading. Yet, clearly, they play central roles. Perhaps one useful way of exploring that is to examine the extent to which the military is unified in outlook -- ideological, cultural, social, and hierarchically -- or divided, most likely between officers and rank and file men, which in turn can reflect social or ideological differences (although there could be other fault-lines, such as religion or ethnicity). Moreover, this can change as the revolution progresses.
In the Russian Revolution of 1917, for example, both officers and men were unhappy with the tsarist government of Nicholas II as the year opened, with discussion of palace revolution emerging among high-ranking officers by the end of 1916 while rank-and-file soldiers (and lower level officers) were alienated by the ongoing war (World War I). Both immediately supported the February Revolution -- indeed a rebellion of rank-and-file soldiers in the capital city garrison played a critical role in toppling the regime-- and the new liberal provisional government.
That unity quickly dissolved, however, not just over the paramount issue of continuing the war but also over social, economic, and cultural matters. It proved impossible for the new (civilian) leadership to use the army to contain growing popular grievances and the rise of extremist parties, and to prevent the revolution from spinning out of control in increasingly radical directions. An unsuccessful attempt at military counter-revolution in July 1917 only worsened things. A second (Bolshevik) revolution followed and civil war soon resulted, during which new armies created along new lines had to be formed (the old one having disintegrated). Putting the issue of the role of the military and military service in broader perspective, Joshua Sanborn has written a fascinating book on the transformation of the Russian army (armies) in the broader revolutionary era of the late 19th century to about 1925: Drafting the Russian Nation: Military Conscription, total war, and Mass Politics, 1905-1925. The book raises many fascinating issues about the interrelations of universal (or broad-based) military service, nationalism, notions of citizenship, and politics that apply to many societies, not just Russia.