Opinion: Liberty doesn’t defend itself — right now, it needs our help.

On the streets of Iran in recent weeks, security forces have killed thousands of protesters demanding basic rights and opposing the state’s authority. That stark image is not meant to equate our politics with theirs, but it poses a question Americans cannot ignore: What does it look like when government violence becomes routine and when institutions are used to intimidate citizens?

In the United States, we haven’t yet seen rifles aimed at large crowds, but we do observe masked federal agents detaining protesters in unmarked vehicles...