Guinea is demanding $36bn in damages from the Democratic Republic of Congo at the International Court of Justice.
It blames DR Congo for the jailing and expulsion of a Guinean businessman, when he tried to recover debts.
They were owed to him by the Congolese state and its state-controlled oil and mineral companies.
Amadou Sadio Diallo lived in DR Congo for more than 30 years, running two shipping and trading companies, before his expulsion in 1996.
This happened just prior to the end of Mobutu Sese Seko's regime when rebels seized control of the capital, Kinshasa.
Guinea - which is acting on Mr Diallo's behalf - contends that corrupt officials in Kinshasa were bribed by the companies to prevent him pursuing his claim for owed money.
On this first day of the case in the International Court, DR Congo has an opportunity to raise its objections to the case.
Firstly, it says that present government of President Joseph Kabila cannot be held responsible for events that took place under the former regime.
Secondly, it alleges that Mr Diallo's companies were themselves part of the cycle of corruption that enveloped the country, then called Zaire, under the authoritarian rule of former President Mobutu.
And thirdly, that a successful claim of this size would easily bankrupt an already impoverished state.