Shalmaneser V, the son of *Tiglath-pileser III, continued his father's policy of ruthless expansion and annexation, thus forcing the vassal states of Syria/Palestine to seek help from Egypt. Hoshea rebelled openly against Shalmaneser, with the result that Samaria was captured and, three years later, in 721 BC, the population was taken away to *Assyria by Sargon II, Shalmaneser's successor; Hoshea was himself cast into prison. Biblical sources (II Kings, 17:4) provide the information that Hoshea had involved Egypt in this conflict by sending messengers to 'So, King of Egypt' with requests for help. Attempts have been made to identify So with an Egyptian ruler but it seems more probable that this was the name of an Egyptian general.
BIBL. Kitchen, K.A. 3rd Int. pp 372-4; von Zeissl, H. Athiopen und Assyrer in Agypten. Gluckstadt: 1944.
Biographical Dictionary of Ancient Egypt by Rosalie and Antony E. David
Ancient Egypt. A Reference Guide. EdwART. 2011.