It seems like a fairly common outcome for a democracy to at some point have a coup, so why hasn't the US? Or is that what the US civil war started as, an attempted coup that became a war? I remember reading somewhere that after the election of 1800 when Jefferson was about to take over, there were many who feared it might turn bloody and breathed a sigh of relief when it was peaceful. Sorry if this is off base, I'm not nearly as knowledgeable about the relationships between a country's military and it's civilian government.
To follow up:
What are characteristics of Governments which tend to produce coups, versus the characteristics of Governments which tend to maintain peaceful transfers of power?