The Cheonan was taking part in a series of rolling exercises called Key Resolve/Foal Eagle, which continued in some shape or form from early March until late April.
Dozens of ships from both the South Korean Navy and USN were involved. Among the fleet were four Aegis ships: the USS Shiloh (CG-67), a 9,600-ton Ticonderoga class cruiser, the USS Curtis Wilbur (DDG-54), a 6,800-ton Arleigh Burke class guided-missile destroyer, the USS Lassen, a 9,200-ton Arleigh Burke class guided-missile destroyer and Sejong the Great, a 8,500-ton South Korean guided-missile destroyer.
Although not confirmed, it would be unlikely that such a gathering would not be supported by USN Nuclear Subs and the ROK's own type 214.