I traveled from Baltimore to join hundreds of thousands of protesters at counterdemonstrations around Mr. Trump's swearing-in. Little did I know that I would be swept up into a legal nightmare that demonstrates how prosecutors intimidate and manipulate defendants into giving up their rights. Minutes after I got to downtown Washington on Jan. 20, police officers used pepper spray, sting-ball" grenades and flailing batons to sweep up an entire city block in a mass-arrest tactic known as "kettling." Next, prosecutors ... took the highly unusual step of indicting more than 200 of those arrested. Most of the people in the group, which includes journalists, legal observers and volunteer medics, face charges of engaging in a riot, inciting a riot, conspiracy to riot and property damage. In addition to seizing the contents of at least 100 cellphones, prosecutors secured broad warrants for Facebook pages. The government has failed to provide most defendants in the case with evidence of their alleged individual wrongdoing. For example, I was offered a plea deal (to a single misdemeanor charge) on the basis of virtually nothing more than being at the site of the protest. This serves to illustrate a critical problem in the American justice system: Prosecutors have the power to single-handedly destroy lives, and there are few consequences for abuse of that power. At the same time, their main measure of success is the ability to secure convictions, not the degree to which justice is served.