the whole country; all the land
Archaic literary term for the entire nation or realm, originally meaning 'within the four seas'. In modern Japanese, 全国 or 国中 are used instead.
The phrase 海内無双 means being peerless throughout the land.
unparalleled in the whole country; peerless throughout the land
From Classical Chinese 海内 (hǎinèi), literally 'within the four seas', referring to the entire realm. The Japanese reading かいだい is the on'yomi of the characters.