What is 'AUKUS' and why it is troubling China?

The US, the UK, and Australia announced the creation of a new trilateral security partnership. 

In a joint statement, the three governments said the partnership, called "AUKUS", will help "significantly deepen cooperation on a range of security and defence capabilities".

The first initiative under AUKUS will be delivering a nuclear-powered submarine fleet for Australia and the three countries will spend as many as 18 months discussing how this capability will be delivered, according to the statement.