During world war i, the american government arrested who publicly opposed the war effort.

The answer is: dissidents

Dissidents refer to the group of people who actively challenge the policy made by  a certain instution (could be a government).

During world war I, amrican government felt that leaving the dissidents free to talk whatever they want could damage public's support toward the government regarding American involvement in world war I. So, the government decided to silent them.

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.

democracy

explanation: