Long but good read on Martial Law in the US. If you are being fearmongered about the issue, it's complicated and not nearly so linear or universal as those talking about it suggest. And it's not anything the US military is prepared to do by doctrine/geography.

https://www.brennancenter.org/our-work/research-reports/martial-law-united-states-its-meaning-its-history-and-why-president-cant