THE REICHSTAG, Berlin — Here in this building, 68 years ago, German democracy died, ushering in the darkest period of 20th-century history.

A whole nation went mad, its noble past forgotten. Thoughtful debate was replaced by crazed slogans — about “the Greater German Reich,” “Lebensraum” (living space) for the German people and, of course, “Juden raus!” (Jews out). The Reichstag itself went up in flames and was never repaired. Adolf Hitler had no need of parliaments. Doctrine and inner voices were enough.

Today, all is democratic calm and peace again, except for the sound of the cranes building the new Berlin all around. The harsh language has softened and the German Parliament again sits in the Reichstag Building, modernized by a British architect and cleansed of its horrific past, except for some preserved panels of graffiti left by Russian soldiers when they entered Berlin in 1945.

Could it happen again? In this age of transparency and global communication, could a whole people first turn in on themselves and then turn ferociously on their smaller neighbors while the democracies stood by, fearful, yet reluctant to intervene or start another war?

Or, to put it another way round, if the Internet and CNN had existed in the 1930s to relay scenes of Nazi aggression nightly into people’s living rooms, would public opinion in, say, Britain or the United States have remained passive and disinterested? Or would there have been a demand for intervention by “the international community” to halt massive violations of human rights and clearly incipient threats to world peace?

The question is highly relevant today, seven decades later, because the concept of humanitarian intervention and warfare is now at the center of international debate. When, and how early, is intervention justified to address human-rights violations? When is outright war against human-rights offenders justified? When can the principle of the inviolability of national sovereignty (hitherto at the heart of the international order) be cast aside by the intervening forces?

Above all, who decides? Who exactly is the international community, and who makes the actual decision to mobilize large numbers of troops, move them here and there, invade this country or that, bomb specific targets and so on?

The war in Kosovo, a sovereign part of Serbia, raised all these issues in their most acute form. The U.N. Security Council could not authorize invasion because China and Russia were resolutely against it. Anyway, the U.N. Charter is founded on the principle of nonintervention in domestic jurisdictions.

The task therefore fell to NATO, acting in the name of 19 democratic nations but led by the U.S., to take preventive military action. For justification, it had to rely on such unspecific claims as evidence of massive human-rights abuses, fears of genocide, a possible destabilizing impact across the whole Balkans and the general feeling, driven by public opinion, that “something had to be done.”

Above all, there was a widespread sense that the Serbian leader, Slobodan Milosevic, was a crazed dictator and that large sections of the Serbian nation had also lost touch with reality. In the last resort, violence had to be met with violence in the name of humanity — however contradictory that might seem. Appeasement was useless.

The action that followed in the spring of 1999 seemed to do the trick. Serbian soldiers and police were driven out of Kosovo and now Milosevic has also been overthrown (although many problems about Kosovo’s future remain). But was it lawful, and does it constitute a precedent for further interventions, with or without U.N. blessing?

The issue is urgent because there is a new player on the scene — the embryonic European Rapid Reaction Force, under the command of the European Union and working with, but sometimes separately from, NATO.

On what principles, and on whose say-so, will this new force be deployed, and against whom? The cardinal lesson of Kosovo, and also of the Persian Gulf War against Iraq in 1990-91, is that nationally organized terror and systematic murder by an unscrupulous regime must be met decisively and swiftly, with all necessary means, even if this means intervening in a state’s internal affairs.

That is just what the democracies failed to do with Hitler in the ’30s — or, for that matter, with fascist Japan. The will was not there, and a largely uninformed public did not care — until it was too late. And it is what the United Nations remains reluctant to do today, despite the urging of its secretary general, Kofi Annan, to rethink the issue.

Will the new EU-led force muster the unity to act promptly in the next crisis, with or, more likely, without U.S. support? Will it match the boldness and resolution either of NATO or of a few individual nation states acting in concert? And will it feel that it has found some new kind of legitimacy in doing so?

These are the next big questions to be answered as this strange new era of interventionist internationalism evolves. Maybe nothing as terrible as the madness of Nazi Germany will ever happen again. Maybe open democracies will never again be derailed by rabid nationalism. But can one be sure?

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