I have a venerable (high spike count) rear wheel that has survived many a year before and after I purchased the bicycle.
Today a drive side spoke failed at the nipple while climbing. The road surface was smooth. I didn’t hit any bumps. I was seated but pushing power at cadence.
The spoke failed within the thread area.
Despite searching online I’ve not found a succinct explanation of the causes and actions of spoke failure at nipple (as opposed to hub).