この条約は恒久的な解決を目指している。
This treaty aims for a permanent solution.
The government is looking for a permanent solution to this problem.
The international community is cooperating to achieve lasting peace.
不変 means something does not change, while 恒久 means something is long-lasting or permanent, often used for peace or institutions.
恒久 is a formal term for lasting permanence, often used in phrases like 恒久平和 (lasting peace), while 悠久 evokes the vast, poetic sweep of time itself.
permanent and immutable; eternally unchanging
permanent measure; long-term solution
permanent establishment (tax law)