Keys to understanding the crisis:

The Civil War.  Burundi endured a 12-year civil war, fueled by tensions between ethnic Hutus and Tutsis, that killed some 300,000 people.  Diplomats fear a repeat if current violence is not checked.

Term Limit Controversy. Burundi's constitution limits the president to two terms.  But President Nkurunziza ran for (and won) a third term this year, setting off violent protests and a failed coup attempt.  

Assassinations. Several military and political figures have shot dead or wounded since August, including the president's chief of security, the former head of the military, and the spokesman for an opposition party.

The Refugees. The UN says more than 215,000 Burundians have fled to Rwanda or other neighboring countries.

Government Intimidation. Diplomats have expressed alarm at government rhetoric and a disarmament campaign that is targeting opposition strongholds.