Just as non-communist Southeast Asia and the USSR were beginning to warm to each other, Vietnam invaded Cambodia in the last days of 1978, an act that ASEAN vehemently opposed. ASEAN viewed Moscow as a strong back of Vietnam. Shortly before Vietnam went into Cambodia, it had joined COMECON and entered into what amounted to a mutual defence treaty with the USSR, whose intentions in the region the ASEAN countries regarded with suspicion. Indeed, a strong motivation for ASEAN opposition to Vietnam's incursion into Cambodia, even if that ended the reign of terror of the Khmers Rouges, was the fear of an expansion of Soviet influence in the region.