Sentence:  fearful of being mistaken for a Jew , he seeks protection in his Nazi uniform ; clinging to the enemy Jew idea , he is forced to overcome habits of politeness and neighborliness ; once in power he <WSD>begins</WSD> to give vent to a criminal opportunism that causes him to alternate between megalomania and envy of those above him . 
Meaning of the word marked with <WSD> tags:  take the first step or steps in carrying out an action