The army is not the problem. The police force is the problem. If an army sides with tyranny, tyranny will win, but armies are not trained in urban suppression, nor are they indoctrinated to believe their own people are the enemy. Quite the opposite in fact. Armies do not arrest and imprison citizens for political crimes. And ordinary soldiers answer ultimately to career military officers who typically reside outside the political structure. In theory, military leaders answer to the civilian leadership, but they are independent enough that they are quite capable of making their own decisions about right and wrong.
Police forces are much more easily purged and molded to suit the political leaders' whims. Hitler installed his tyranny by taking control of the police, not by subverting the military. Stalin exerted his control over the Politburo by framing opponents for crimes and using the police to administer extrajudicial punishment (usually execution after a show trial). The Great Purge was executed by the NKVD, which was the Soviet secret police. In fact, the Red Army itself was purged, which of course proved to be a disaster for the Soviet Union in 1941 when the Wehrmacht ran roughshod over the poorly led Soviet forces for several months.