The fuel has to be loaded into the rocket just before launch. Solid fuel rockets are set to go when needed. It takes time to fuel those missiles, a good deal of time I believe.
I beg to differ. Liquid fuel missiles use storable, hypergolic fuel and oxidizers. Old Atlas ICBMs used LOX and had to be loaded prior to launch. When the Titan II ICBM came online in 1962 we had a liquid fuel missile that was ready to launch in a moments notice. The only thing that would signal a launch was when the Silo Closure Door opened. As soon as that door cleared the Launch Duct the Fire Engine signal would be sent from the launch computer and the engines would fire.
On a Titan II ICBM the total time of the entire launch sequence was 52 +/- 5 seconds.
All Soviet liquid fuel missiles starting in the late 60s copied the Titan II system and were of the storable variety.
One of the big reasons we went to solid fuel (Minuteman and Peacekeeper) was the relative safety of the solid fuel compared to the very hazardous liquid fuel and oxidizer. The solid fuels do not have as large a Specific Impulse as the liquids and the Russians want to launch a large number of very big nuclear devices so they are still messing around with the liquids. Again. These liquid fuel and oxidizers are stored directly on board the missile and do not have to be loaded just prior to launch.