Vietnam's military, which defeated three of the world's mightiest armies, plans to halve its size to cope with the daunting challenges of peace. As Hanoi goes ahead with its withdrawal from Cambodia and rapprochement with historical enemy China, entire divisions are being demobilized or diverted to civilian roles, officials said on a recent visit. Gen. Tran Cong Man, editor of the Army newspaper, said the 1.1-million-member People's Army of Vietnam would be cut to about 500,000 when normality returns to Vietnam's long borders with China and Cambodia. He did not say how long it would take to halve the army, but other officials have said 300,000 would leave the force "in the next few years."