When Britain’s next prime minister takes office on July 5, after he is finished with the photo opportunity outside Number 10, he will sit down at a desk to write sealed letters to the commanders of the country’s nuclear missile submarines.

It is the first thing every prime minister does, and in happier times it would just be part of the ritual of British politics.

But these are not ordinary times.

The Bulletin of the atomic scientists Doomsday Clock, an index of the perceived danger of nuclear war, is set at 90 seconds to midnight - the closest it has ever been.

The United Nations Secretary General this month has warned we are on a nuclear “knife-edge.”

How have we got to this point?  What can be done to avert disaster? Or is nuclear apocalypse now inevitable?

In this episode of Defence in Depth, Roland Oliphant, a senior foreign correspondent at The Telegraph, explains the return and rise of nuclear diplomacy in the 21st century. 

Watch Roland’s video analysis above. Find more episodes of Defence in Depth on The Telegraph’s YouTube channel.