Granting national courts' jurisdiction over other countries would prompt would-be plaintiffs to "shop around for the most favorable national courts" and would lead to "legal disorder," said Germany's top legal adviser.
Germany says its sovereignty was violated when Italy's Supreme Court ruled in 2008 that an Italian civilian, Luigi Ferrini, was entitled to restitution for his deportation to Germany in 1944 to work as slave laborer in the armaments industry.
Germany has paid tens of billions of dollars in war reparations under various international and bilateral agreements since the 1950s.
If the Italian decision stands, "the consequences would be severe," said Susanne Wasum-Rainer, Germany's director-general of legal affairs. The postwar system for reparations "would be put in question and open to challenges before domestic courts."
Italy says violations of humanitarian law supersede other laws and international accords. No immunity can be accorded to crimes against humanity, it says.
The Italians courts ordered the seizure of German property to enforce the reparations decision.
Rome's case is being supported by Greece, whose citizens have similar claims against Germany, and will be given a hearing during the week of arguments before the 15-judge World Court.
In 2008, the Italian court decided on the seizure of Villa Vigoni, a German-Italian cultural center on Lake Cuomo, to "enforce" the claims by Italians and Greeks seeking compensation. Germany's protest against the seizure formed part of its appeal to the World Court against Italy.
'Matter of justice'
The Greek case involved residents of the Greek village of Distomo, where Nazi troops killed 214 civilians on June 10, 1944 - one of the worst atrocities in occupied Greece.
"It's not a matter of money. It's a moral thing, to find justice," said Miltiadis Sfountouris, who said his 78-year-old mother jumped from a window to escape the Nazi death squads. Still a resident of Distomo, she has never received indemnities, said Sfountouris, the secretary general of the Hellenic Association of Victims of German Occupation.
It could take months before the court delivers its decision.
Germany said the court must focus on the implications of its ruling rather than on the individuals.
"What this case is not about is the Second World War, violations of international humanitarian law or the question of reparations," said Wasum-Rainer. "International law would be atomized and, of course, politicized" if the court endorses Italy's position.
Germany says the Italian precedent would open floodgates for restitution claims by individuals around the world, a situation it tried to avoid in negotiating reparations accords with Israel and with countries that had been occupied during the war, and with specific groups such as the Conference on Jewish Material Claims against Germany.
Under one recent program, Berlin paid nearly $6 billion to 1.6 million people or their relatives who were exploited for slave labor during the war.
Since the Ferrini case, dozens more claims have been filed in Italy, and about 80 cases are pending in the courts.
Germany says Italy agreed to waive further national claims against Germany in a 1947 German-Italian peace agreement, including claims by its citizens.
A ruling for Italy now would mean "all interstate peace settlements concluded after an armed conflict would be put into jeopardy by allowing domestic courts to re-examine and to reopen them," the German advocate said.
If some victims were left uncompensated and felt the reparations were inadequate, "we do regret this," said Wasum-Rainer. "However, reparations are not the subject of this case."
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