The war was initiated by Pakistan, who thought after the defeat of India by China in 1962, that Indian military would be unable or unwilling to defend against a quick military campaign in Kashmir, and because the Pakistani Govt. was becoming increasingly alarmed by Indian efforts to integrate Kashmir within India. There was also a perception that there was widespread popular support within for pakistani rule and that the Kashmiri people were disatisfied with Indian rule.
After Pakistan was successful in the Rann of Kutch earlier in 1965, Ayub Khan (by nature a cautious person) was presuured by the hawks in his cabinet (led by Z.A.Bhutto)and the army to infiltrate the ceasefire line in Kashmir.The action was based on incorrect premise that indegenous resistance could be ignited by a few saboteurs. Ayub resisted the idea as he foresaw India csossing the international frontier in retaliation at a point of its choosing. The Bhutto faction, which included some prominent generals , put out canard that Ayub's cowardice stemmed from his desire to protect his nearly acquired wealth, it was boasted at the time that one Pakistani soldier was equal to four Indian soldiers and so on.On Aug 5, 1965, between 26,000 to 33,000 Pakistani soldiers crossed the line of control dressed as Kashmiri locals headed for various aras within Kashmir. Indian forces, tipped off by the local populace, crossed the ceasefire line on Aug 15, 1965.
The initial battles between India and Pakistan were contained within Kashmir involving both infantry and armour units each country's air force playing major roles. It was not until early Sept. when Pakistani forces attacked Ackhnur that the Indians escalated the conflict by attacking targets within Pakistan itself, forcing Pakistani forces to disengage from Ackhnur to counter Indian attacks.