"This is a historic development" for international justice, says Yves Sorokobi, spokesman for Mr. Moreno-Ocampo.
Announcing the launch of the investigation Monday, Moreno-Ocampo called for "sustained cooperation from national and international authorities."
That is unlikely to be forthcoming from Sudan, whose government has said it will not hand over any of its citizens for trial in the ICC. Sudan has signed, but not ratified, the ICC treaty. Thus it is not party to the court, but is subject to its jurisdiction because the UN Security Council referred the situation in Darfur to the court.
Some top Sudanese government officials, along with rebel and militia leaders, are believed to be on the secret list of 51 people suspected of "grave international crimes" by a UN commission of inquiry that reported last October.
That will pose a challenge to the ICC investigators gathering evidence in Sudan to support any indictments, since most suspects and victims are under the Khartoum government's control.
And the regime has ways of making things difficult. After UN Secretary-General Kofi Annan's recent visit to Darfur, the government harassed his interpreter, who had helped him speak to rape victims.
Investigators will also face a very dangerous security situation in Darfur, where aid convoys are regularly attacked. Unlikely to want to rely for protection on the very Sudanese forces they are investigating, they will not be able to turn either to the African Union (AU) troops in Darfur, because their mission is to monitor a cease-fire, not to protect foreign nationals.
The Sudanese government says its own courts are capable of trying war crimes, and has recently begun to overhaul its justice system.
A recent Amnesty International report, however, found "no evidence that Sudanese courts are able to investigate and try these crimes by both sides independently and fairly," says Mr. Hall.