The first verdict of the International Criminal Court (ICC) is expected on Wednesday in the trial of Congolese militia leader Thomas Lubanga.
Professor David Scheffer, who served as the first US ambassador-at-large for war crime issues, told the Today programme's John Humphrys that the court is still taking "baby steps".
He went on to say that the court is aimed at bringing leaders of atrocities to justice which is "far more complex" than normal cases.
"We've only been at this for 17 years and lessons are being learned at all times", he said, including this current case of Thomas Lubanga which was delayed by a year and a half because of complications during the trial.
On the effectiveness of the court, he said that there is divided opinion on whether the ICC can be fully effectively unless the United States signs up to it.
It would be "tremendous if US could join more rapidly", he said, but maintained that it is in some ways already a member because of the amount of assistance it provides.
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