Britain and Rwanda face off at the Permanent Court of Arbitration in The Hague over a scrapped migrant deportation deal.
Kigali seeks more than £100 million it says London still owes, alongside a formal apology for ending the controversial scheme.
The hearing, lasting three days, pits officials from both countries before a three-judge panel to resolve contractual disputes dating back to 1899.
The UK government terminated the deal after the Supreme Court deemed it illegal, calling it politically and legally untenable from the start.
Former Prime Minister Boris Johnson signed the 2022 agreement to send migrants arriving in Britain via boats or lorries to Rwanda.
Only four individuals were sent under the plan, all voluntarily, while about £290 million had already been transferred to Rwanda, UK officials said.
Rwanda claims the UK still owes two £50 million payments and an additional £6 million for housing Congolese refugees, citing breached obligations.
The African nation accused London of abandoning commitments simply due to a change in internal political priorities, calling itself “rightly aggrieved.”
Britain countered that Rwanda’s claims are legally weak and politically motivated, linking them to aid cuts over Kigali’s support for M23 rebels in DRC.
The dispute highlights tensions between the nations, with trust eroded after the UK suspended most financial aid in response to Rwanda’s actions in Congo.
Rwandan officials are set to present their arguments on Wednesday, Britain will respond Thursday, and both sides will sum up on Friday.
The tribunal’s ruling is expected to take several months, amid Britain’s ongoing debates over immigration policy post-Brexit.