Uganda's crackdown on ADF rebels in eastern DR Congo, a mission launched jointly with Kinshasa, was driven in part by a desire to protect Ugandan oil interests, researchers said on Monday.
But the joint operation also "rankled" Rwanda and became a factor in the resurgence of another rebel group, the M23, which DR Congo says is being supported by Kigali, they said.
The report, by the Congo Research Group (CRG) at New York University and Ebuteli, a Congolese research institute, delves into the unprecedented crackdown, Operation Shujaa, which the two countries launched in late November 2021.
Ugandan troops crossed the border in a joint operation with Democratic Republic of Congo forces that targeted strongholds of the Allied Defence Forces (ADF) -- a rebel group blamed for thousands of deaths in eastern Congo and a string of bombings in the Ugandan capital Kampala.
Military officials on both sides have proclaimed Operation Shujaa to be a success.
But the report says the sketchy information available suggests the ADF have been fragmented into "three or four clusters" and moved further inland away from the DRC-Ugandan border.
The operation "has not managed to structurally weaken the ADF," it says, describing the rebels as a "resilient, mobile and adaptive" foe.
From Uganda's viewpoint, the military operation was also intertwined with other goals, including the securing of Ugandan oil projects around Lake Albert that lies on the border between the two countries, the report says.
The multi-billion-dollar scheme counts on investment by foreign majors to exploit reserves under the lake and build a pipeline to get the oil to market.
"Ugandan intervention has also created a potentially explosive regional situation with Kigali," the writers warn.
"The current military operation takes place in a context of already tense relations between Uganda and Rwanda, driven by security concerns and access to minerals in eastern Congo. The operations and the road infrastructure (built under Operation Shujaa) expand Uganda’s sphere of influence in the region and could potentially lead to a further escalation of these tensions."
Tensions have flared in recent days between Kinshasa and Kigali over the M23, which like the ADF is one of scores of armed groups that roam eastern DRC, many a legacy from two regional wars in the late 1990s.
The primarily Congolese Tutsi militia briefly captured Goma in 2012 but a joint offensive by UN troops and the Congolese army quelled the rebellion.
The group resumed fighting in November last year after accusing the Congolese government of failing to respect a 2009 agreement under which the army was to incorporate its fighters.
Thousands of people have fled into Uganda and the DRC territory of Rutshuru since a second bout of violence flared in March.
"The Ugandan intervention has already had far-reaching geopolitical repercussions. The operation has rankled Rwanda and was one of the reasons for the reemergence of the M23 rebellion," says the report.
The DRC has accused Rwanda of helping the M23's resurgence -- a claim that the Rwandan government has denied -- and in recent days both sides have accused the other of carrying out cross-border strikes.
On Monday, local sources said the M23 had overrun the trading town of Bunagana, on the border with Uganda.
Bilateral relations between Rwanda and its vast neighbour to the west have a long history of strain, beginning with the mass arrival in eastern DRC of Rwandan Hutus accused of slaughtering Tutsis during the 1994 Rwanda genocide.
"The re-emergence of the M23 did not come out of nowhere," Jason Stearns, the director of CRG, said in an email.
"It was triggered by various factors, including geopolitical shifts brought about by the Ugandan intervention."