US Sub v. Russian Warships

According to this report, Russia has chased a US sub out of her boundary waters:

http://en.ria.ru/russia/20140809/191869986/Russia-Forces-US-Submarine-Out-of-Boundary-Waters.html

All current US subs are nuclear. If this one was a Virginia-class sub, it looks like this:

http://en.wikipedia.org/wiki/Virginia-class_submarine

I was a nuclear submarine officer in a former life, a real steely eyed killer of the deep.

Subs like this can spy as well as fight. And if one of them were to be sunk by the Russians, it would definitely start a war.

I wonder what her rules of engagement were?