Amin's Rule in Uganda Imperiled As Tanzanian Force Nears Capital
Amin faces gravest armed threat to regime since he seized power 8 yrs ago; invading Tanzanian troops have seized major cities in southern Uganda and are marching on Kampala; Ugandan exiles are slipping back across border in significant numbers for 1st time in yrs to take up arms and engage in sabotage; Tanzanian Pres Nyerere denies he is trying to bring down Amin; emphasizes that his troops crossed border in retaliation for Ugandan invasion and are fighting only to insure that another invasion does not occur; illus (M)