Nuclear-armed US submarines that went more than a decade without calling on foreign ports due to post-9/11 security concerns are once again visiting other countries.
A stop in September by USS Wyoming at Faslane in Scotland was the first of what are expected to be occasional visits to foreign ports.
Michael Connor, a retired Navy vice admiral who served until September as commander of the American submarine force, said the change reflects a desire to emphasise that the submarines are all over the world and also to give the crews an experience that is open to sailors on virtually all other Navy ships.
"The fact that a port visit is a possibility, even if it can't be delivered on every patrol, that is a big deal to the sailors. I know it was a big deal to me," Mr Connor said.
He said port visits such as the recent stop in Faslane also promote professional development by reinforcing a crew's ability to navigate and resupply their sub anywhere in times of crisis.
The Navy has 14 submarines with nuclear warheads that roam the oceans as part of the US strategy to deter an enemy strike.
The crews that operate the Ohio-class ballistic-missile submarines, also known as "boomers", typically deploy for 70 to 80 days at sea with limited opportunities to surface for training.
The smaller attack submarines, in contrast, deploy for roughly six months with about four port visits, some for maintenance and others for "liberty" or crew morale.
Mr Connor said retention rates for sailors on ballistic-missile subs are among the highest for Navy organisations, but lengthy internal Navy deliberations concluded the port visits are an important incentive.
"It's a huge motivator," he said. "It's a reason people want to be in the Navy. It's a reason people want to be up to date on their qualifications so they're allowed to go ashore and take this time."
While port visits took place occasionally in the 1990s, several factors led to them coming to a halt after 2003.
Mr Connor said the thinking after the September 11 attacks was that the subs were too special to assume any risk at all. In addition to security concerns, a spokesman for the submarine force, Navy Commander Tommy Crosby, said there has been limited flexibility in scheduling as the number of ballistic-missile subs is down from the 18 the Navy had in the 1990s.
The primary reason for the Scottish visit, Cmdr Crosby said, was "to strengthen co-operation and interoperability between the US and the UK and to demonstrate our capability, flexibility and continued commitment to our allies".