It is thrilling to watch the destruction of physical barriers to freedom. But equally gratifying and perhaps more important practically are those moments marking a dramatic shift in the perception and policy of nations. Two of these occurred this week. They are worth remembering.

In South Africa, President F.W. de Klerk announced the end of a national state of emergency that was nothing more than a four-year-long excuse for the white minority to avoid dealing directly with the political and social demands of the majority blacks.

In Moscow, the leaders of the Warsaw Pact began the serious work of ideological metamorphosis — entering their meeting as a military alliance, they emerged as an economic association that eventually could be absorbed into a larger European entity.

These transformations have important things in common. Although they appeared abrupt, both are steps in gradual processes that are immense in human terms and inevitable in the march of history. In neither case can there be any successful effort to permanently turn back the clock.