pillar of state; mainstay of the nation
A formal, somewhat literary term for a person or institution that is a fundamental support of the country.
He is respected as a pillar of the state.
柱石 alone means 'pillar' or 'cornerstone' in a general sense; 国家の柱石 specifically refers to a pillar of the nation.
Compound of 国家 (nation) + の (possessive particle) + 柱石 (pillar, cornerstone). The exact historical derivation is uncertain; the expression is used figuratively.