He was mayor of Cologne from 1917 to 1933 and witnessed the rise of the nazis from that position. As a conservative, he shared their opposition to communism and initially thought they could be "tamed", but he despised them nevertheless and was horrified when they seized power in 1933. When newly-elected Hitler came to Cologne in the same year, mayor Adenauer refused to welcome him and had all swastika flags removed that the nazis had put up on the city's many bridges in the night before. This earned him the reputation of being one of the nazis' last major public opponents and certainly contributed to him being elected chancellor after the nazis were ousted. Back in 1933, however, they had him removed from office and tried to lynch im a few months after that incident.
Have a listen:
https://youtu.be/Ghh9iZz5BHQ