What are our obligations as Jews when we witness someone planning to commit a crime against the government?

When witnessing someone planning a crime against the government, your halachic obligation depends on the crime's nature. For crimes involving direct physical harm (e.g., murder, rape), you must immediately report to authorities; this is an absolute mitzvah. However, for monetary or non-life-threatening issues, you must first bring the matter to a Beit Din. Only after the accused rejects the Beit Din's process three times and is put in cherem may the Beit Din permit you to approach secular authorities. Failing to follow this procedure for lesser offenses is a severe "din moser" violation, risking one's share in Olam Haba. In essence, act immediately for threats to life, but for lesser crimes, exhaust Torah channels through the Beit Din first.