The International Court of Justice has begun hearing Serbia's case against Kosovo.  Judges in The Hague have been asked by the U.N. General Assembly to rule on the legality of Kosovo's 2008 declaration of independence.

Lawyers for Serbia argued that Kosovo's move not only challenged Serbia's sovereignty, but undermined international law.

Harvard Law Professor Andreas Zimmermann is one of the lawyers arguing Serbia's case in court.

"With all due respect to the actors involved, it was certainly not helpful - to say the least - for the mediator appointed by the [U.N.] secretary-general to refer publicly to Serbia as a thief that had stolen Kosovo from the Albanian Kosovo population, as if Serbia did not have a valid title to the territory ever since 1913," Zimmermann said.

And even before that, say Serbia's lawyers, Kosovo had been the "historical cradle of Serbia," one of the "essential pillars of its identity."

But Kosovo Foreign Minister Skender Hyseni told the court it would be "inconceivable" to reopen negotiations with Serbia on Kosovo's future.  He said that "would be highly disruptive, and could even spark new conflict in the region."

Ten years ago Belgrade attacked separatists in what was then its southern province, killing about 10,000 Kosovo Albanians and displacing close to a million more.  NATO bombed Serbia for 78 days.  The United Nations then set up a provisional government in Kosovo.

But when the region declared independence last year, which Serbia says violated the U.N. and thus international law, 63 nations recognized Kosovo's sovereignty.  The United States and most European Union members are among them, although Russia and the U.N. Security Council are not.

Many of those countries will make their own arguments before the court during the next nine days.  While judges are not expected to rule on the legality of Kosovo's move for months, many countries that have their own problems with breakaway republics are closely following this case.