The FLN is the armed wing of the Rwandan Movement for Democratic Change, a political opposition group founded by Paul Rusesabagina, the hotelier whose actions during the 1994 genocide against the Tutsi were depicted in the Hollywood blockbuster Hotel Rwanda.
In December last year, Nsabimana claimed responsibility for setting fire to passenger buses in Nyungwe Forest—a region popular among tourists coming to see endangered mountain gorillas—which led to the death of two people and many injuries.
Those attacks prompted many Western governments including France, Germany, Canada, and Australia to advise their nationals against travel to the area.
A number of rebel groups opposed to Kagame have proliferated just over Rwanda's borders in remote forest regions of DR Congo and Burundi.
The FLN is also affiliated with the Democratic Liberation Forces of Rwanda (FDLR), another rebel group based in DR Congo, which occasionally carries out cross-border attacks against Rwandan forces.
In his guilty plea, Nsabimana admitted to collaborating with Burundi's intelligence services and Uganda's military.
"We asked them for military and diplomatic support against Rwanda, and they were willing to help us," he said.
Kagame has accused his neighbours of meddling in Rwanda's affairs and sponsoring armed movements against his government in Kigali.
In February, he ordered the border with Uganda shut, and ties with Burundi have long been sour.