The Treaty of Westphalia as a Permanent Legacy of Modern International Relations

Why do most historians of international relations view the Treaty of Westphalia as a permanent legacy of modern international relations?

Consider major international relations events in the centuries since Westphalia – the French Revolution & the Rise of Nationalism, the Concert of Europe (balance of power) from 1815-1914, the concurrent onset of modern European colonialism during the same period, the breakdown of peace with WWI & II, the Cold War, and finally, the post-Cold War present. From this wider horizon, identify at least three key factors or elements that appear to influence whether the system becomes stable or unstable.