Original: From 1947 to 1948, soon after the Partition of India and of the Indian Army, the two new armies fought each other in the First Kashmir War, beginning a bitter rivalry which has continued into the 21st century.
Simplified: Soon after the Partition of India, both the newly formed armies fought each other in the First Kashmir War from 1947 - '48 which begun the bitter rivalry that has continued into the 21st century.