A country that wants to have nuclear weapons as a part of its defense forces, has to also build up a lot of accompanying infrastructure. Along with having nuclear weapons, it needs a delivery system and a C3 I system (Command, Control, Communications, and Intelligence, see below). Apart from this hardware, it needs to formulate a nuclear doctrine and strategy. All this constitutes weaponization.
Nuclear weaponization is a long process. It starts with the designing and testing of the weapons and delivery systems. The next step is the large-scale production of these systems. Along with this, the C3 I has to be set up. The final step is the induction of these systems into the armed forces, the training of the personnel and finally the deployment of the weapons.
A delivery system is the means by which the nuclear weapon is “delivered” to the victims. It could consist of aircraft carrying nuclear bombs and missiles with nuclear warheads. These missiles could be either launched from land, ships or submarines. Some tactical nuclear weapons are small enough to be made into artillery shells and fired from cannons.
Missiles are the preferred means of delivery for strategic weapons since it is virtually impossible to set up a system of defense against them. They travel so fast (several miles per second, the speed of a bullet) that there is no way to reliably detect and shoot them down before they hit the target.
C3 I stands for command, control, communications and intelligence. When a country has nuclear weapons and a delivery system, it automatically constitutes a nuclear threat to other countries and thus increases the chances of being itself subject to a nuclear attack. Since there is no known defense against an attack by nuclear-armed ballistic missiles, the country has to set up a complicated system of radars and satellites, or Early Warning Systems, to keep a constant watch for an enemy missile attack.
Early Warning Systems are linked via a highly automated network of computers and communication systems to its nuclear weapons and the leaders authorized to launch a nuclear attack (the National Command Authority). All warnings are evaluated and passed up the chain of command if they are believed to be a real attack.
Launch orders have to be conveyed quickly and reliably to the personnel who are actually manning the weapons. A procedure to do all this has to rely on a reliable and robust communication network. This procedure has to also ensure that no weapon is fired without proper authorization or by mistake. This is the command, control and communications aspect.
All this together constitutes the C3 I system.