I'm not entirely sure about this, I'm running purely running off people I know who are going into this, but if you want to teach, it depends on where you want to teach. Many schools K-12 want at least for you to have a BA in teaching, if not a masters in teaching, but like I said I'm not sure about this. If you want to go for a masters of history, then you can teach at a community college. Now, for college itself, in 99% of cases you do need your Ph.D. if you want to teach at that level. But with the k-12 bunch, I would say look at the school districts that you might want to teach in and see how much/ what type of grad education you might need.