What happens if you solder without flux?

I'm curious to understand the consequences of soldering without flux. Could you elaborate on the potential issues that may arise? Is it a crucial step in the soldering process, or can it be overlooked? What are the chances of encountering issues like poor conductivity, weak solder joints, or oxidation without the use of flux? Additionally, how does flux assist in the soldering process, and what are the alternatives, if any, to using flux? Your insights would be greatly appreciated.