national policy; fundamental national principle
Refers to a nation's fundamental guiding principle or long-standing policy, often used in political or historical contexts.
This national policy has not changed for many years.
The new government adopted peaceful diplomacy as its national principle.
That country has adopted pacifism as its national policy.
The decision was criticized for going against national policy.
The new government proclaimed independence as its national policy.
方針 is a general guideline or direction that can apply to any organization, while 国是 specifically refers to a nation's core, long-standing principle.
国税 (national tax) sounds very similar to 国是 (national policy) but refers to taxation, not guiding principle.
Compound of 国 (country) and 是 (right, justice, this), where 是 carries the meaning of 'correct principle' or 'what is right'.