yup..that I know.but that doesn't point replacing army was the reason.JRB was acting more like secret police than an army.and it was created in 1972.so,Mujib had enough time to replace Army if needed.but yes,that time,Army was largely neglected.
war finished on 16th december 1971, so obviously it took time to form JRB in 1972, and BTW all Bangladeshis knows that Mujib never liked army
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GR!FF!N more info
Rift with Military Forces
The number of Jatiyo Rokkhi Bahini personnel increased to 30 thousand by 1975. This large force was armed to teeth. They were given automatic weapons, steel helmets, jeeps, trucks etc.[3]
They had complete uniform with leather boots and a complete table of organization.
On the other hand Bangladesh Army, which was formed through a war against an army of occupation with some true patriots in 21 November 1971, found themselves as an orphan.
While the Jatiyo Rokkhi Bahini was being enlarged, Bangladesh Army was just watching that. There was no T.O.E. (Table of Organization and Establishment).
Military personnel did not have sufficient arms. Most of them had no uniforms, boots, helmets etc. In winter they had to guard the border in slippers.[3]
Major General Manzoor, a freedom fighter who escaped from Pakistan, defying the Army to join the liberation war said Mascarenhas that sometimes military personnel were killed by National Defence Force (JRB) blaming them as collaborators.[3]
All these discrimination created rift between the two forces.
http://en.wikipedia.org/wiki/Jatiyo_Rakkhi_Bahini#Rift_with_Military_Forces
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GR!FF!N JRB formed by RAW, so now you get the picture why we think India is against our armed forces