It involved the Japanese and the Chinese navies, and was the largest naval engagement of the First Sino-Japanese War. The Yalu River is the border between Korea and China. This battle was actually fought at the mouth of this river, in the Korea Bay (or the Yellow Sea, depending on who you ask).
A Japanese fleet under Admiral Sukeyuki Ito was attempting to disrupt the landing of Chinese troops protected by a fleet under Admiral Ting Ju ch’ang. It’s considered a significant beginning for later battles, the so-called dreadnought technology.
“On 16 September the Japanese Navy, having carried out a landing operation at
Chinnampo, was returning to sea. Admiral Ito had with him a powerful force
comprising ten cruisers, a gunboat, an armed merchantman and a flotilla of
torpedo boats. Ito’s flagship, the 4,277-ton Matsushima, mounted one 12.5-inch
and eleven 4.7-inch guns, as did her sister ships Itsukushima and Hashidate. The
Fuso and Takachiho carried two 10.2-inch and six 5.9-inch, the 2,200-ton Hiei
one 10.2-inch and two 5.9-inch and the 2,450-ton Chiyoda ten 4.7-inch guns.
Togo’s Flying Squadron, the Naniwa, Yoshino, and Akitsushima, were also in