"The security environment today is complex. (We) need to be vigilant all the time," Parrikar said at a tri-services seminar here to commemorate the 1965 India-Pakistan war.
Army chief General Dalbir Singh said future wars will not give time to prepare, and hence the military should always be vigilant.
Parrikar said Pakistan paid for its "misadventure" in 1965, and said India clearly won that war although skeptics called it a draw.
The general said frequent ceasefire violations and infiltration bids by militants from Pakistan kept the border in Jammu and Kashmir active.
He said while new methods were being used to create unrest in Jammu and Kashmir, recent examples showed that there were attempts to extend the disturbances to other areas of the state as well.
"Future wars will offer little time for preparation. There will be limited warning time. We need to maintain high operational preparedness at all times," General Singh said at the seminar.
Vice President M. Hamid Ansari, Chairman of Chiefs of Staff Committee and Air Force Chief Air Chief Marshal Arup Raha and Navy chief Admiral R.K. Dhowan were also present at the inaugural of the two-day seminar that will analyse the 1965 war.
India is commemorating 50 years of the war on a grand scale.