Kampala, Uganda — Most of the forces loyal to former dictator Idi Amin have retreated into Zaire and Sudan after their eight-day invasion of northwest Uganda, according to reports reaching here.Tanzanian and Ugandan troops with artillery and light tanks launched a counteroffensive earlier this week in the western Nile region bordering Sudan and Zaire. Ugandan leader Paulo Muwanga said an early morning attack by the invaders took defenders by surprise but swift reinforcements stemmed the advance. The invaders were reported to have been welcomed by some people in the region -- the homeland of Amin's Kakwa tribe. Mr. Muwanga said the attack was carefully planned and launched from Zaire and Sudan. Both states denied involvement.