The 2.5 million strong BIA was demobilised within 2 years of end of ww2.

India and pakistan within two months of independence into war over kashmir.

Pakistan allowed Major akbar khan to assemble pashtun raiders to help the islamist rebellion in Jammu against dogra rule and potentially grab kashmir as whole.
Demobilised muslim soldiers played a huge part.Pakistani officers participated unofficialy.

It took until may 1948 for pakistan army to make an official appearance and few more months for pakistan to admit it.

The curious thing is british played a role as both armies had british c in c and relied heavily on british advice. The problem was indian officers were around rank brigadier at best
at the end of ww2.

Higher command was an issue. Dependency on british for resources like oil and spares were total. This allowed british to allow the new seperated siblings to squabble and
at the same time prevent an full scale war as their interest desired.

There was care taken not to allow IA too close to the pak border past the jhelum river. Reinforcements denied at times. Pakistan was able to move manpower from punjab
to kashmir (with the confidence of carefully given official leaks) at enormous risk.

Nehru wanted to attack pakistan itself like Shastri did in 1965.Mountbatten in desperation (to enforce the british policy of preventing an inter dominion war) promised help on an offensive if india could refer the issue to UN.

India referred the case to UN , not because nehru expected anything great but it was to exist all peaceful options before an assault on pakistan as is advisable diplomatically.

The help never came. The british representative at the UN Noel Baker played a partisan role due to his personal belief in religious unity and british self interest in the islamic world.
USA deferred to britain in a region where they had less experience .

Around the end of 1948, when india was running on a high dislodging the raiders in zojila pass,kargil and relieving poonc, Nehru again asked the british c in c the cost of driving
raiders/PA totally out of kashmir.

The northern areas was always goin to very difficult with 30 peaks > 10000 ft..As for POK, the reply was only an advance to domel was possible . or quick decisive blow, an attack on pakistan was required.

ofcourse british help wont be forthcoming on an attack across the border. In pakistan, the view conveyed was they would certainly lose.

Thus both countries settled for a best bargain of that time.

In 1965,Pakistan tried a land grab again. This time a full scale invasion of pakistan was done in retaliation.