Nearly all spectrum that is usable for commercial telecom operations was earlier allocated to the ministry of defence. When Telecommunication started, the need for spectrum arose and then MoD had to release spectrum in phases to the DoT for commercial deployment. The 1900 MHz Spectrum was just one single swap of 15 MHz in exchange for equal quantity of 2100 MHz spectrum. Apart from that MoD has 10 MHz in 700 MHz band, 20 MHz each in 1800 and 2100 MHz band, 2.5 MHz in 850 MHz band and 20 MHz in 2300 MHz band. So don't worry the defence ministry has plenty of spectrum to run their services smoothly.