Picture the scene:
It's the morning of January 20, 2017. The inauguration ceremony is in full swing, with songs and speeches the warm up act before the main event - the swearing in of Donald J. Trump as the 45th President of the United States, scheduled for 12.00pm - the moment when his predecessor, Barack Obama's term expires.
At 11.45am the inauguration is interrupted with some shocking news. An enemy power has launched a full scale surprise attack on the continental United States. The missiles, thought to be nuclear armed, will reach their targets in 30 minutes. The military is on standby waiting for their orders to come through. But who gives the orders?