You are right and I misremembered. I think mowing down "entire cavalry regiments" is something I read about the Second Boer war where indeed British cavalry charged against Boers equipped with machine guns with disastrous results, whether "entire cavalry regiments" is an exaggeration or not. However, I would say that already by the time of the American civil war traditional cavalry tactics had been proven to be ineffective. American generals learned their lessons and changed their use of cavalry during the war. European generals did not and continued to emphasize the use of sabres and lances. Thus, the early stages of the First world war featured some cavalry charges which ended in blood baths for the charging side.