What are US nuclear submarines capable of?

Russia has downplayed Donald Trump's orders to move nuclear submarines closer to Russia, warning against nuclear rhetoric.

Last week, Trump ordered two submarines to "be positioned in the appropriate regions" in response to what he called "highly provocative" comments by former Russian President Dmitry Medvedev.

But what are US nuclear submarines capable of, and how do they stack up against the Russian fleet? The Security Brief’s Mikey Kay, a former Senior Officer in the British Military, and Bob Fancy, a former Submarine Commanding Officer in the British Navy, explain.