On 7 December 1975, when Indonesia invaded and occupied East Timor, Taur Matan Ruak took to the hills with the recently formed FRETILIN Army, FALINTIL. As a combatant, he participated in battles against the Indonesian military in Dili, Aileu, Maubisse, Ossu, Venilale, Uatulari and finally in Laga on the northeastern coast, where he eventually stayed. Major-General Ruak's first official FALINTIL appointment was at the end of 1976. From 1976 to 1979, he rose through the FALINTIL ranks in the two eastern military sectors, the Central East Sector and the Eastern Point, or the Ponta Leste Sector. Then he became a company commander.