Being able to build a device that can produce a nuclear explosion is a significant challenge in itself. Also challenging is building such a device in a self-contained way which does not require difficult last-minute assembly, and which can be stored in a usable state for years. The first American bombs certainly did not meet this standard.
Captain William Parsons, a U.S. Navy weapons expert with the 509th Composite Group (the B-29 squadron that dropped the atomic bombs on Japan during WWII) described the complex and hazardous operation, in a letter intended to convince his superiors that dummy devices were required for practice runs:
It is believed fair to compare the assembly of the gun gadget [the uranium bomb] to the normal field assembly of a torpedo, as far as mechanical tests are involvedâ€¦ The case of the implosion gadget [the plutonium bomb] is very different, and is believed comparable in complexity to rebuilding an airplane in the field. Even this does not fully express the difficulty, since much of the assembly involves bare blocks of high explosives and, in all probability, will end with the securing in position of at least thirty-two boosters and detonators, and then connecting these to firing circuits, including special coaxial cables and high voltage condenser circuitâ€¦ I believe that anyone familiar with advance base operationsâ€¦ would agree that this is the most complex and involved operation which has ever been attempted outside of a confined laboratory and ammunition depot.
Rhodes, Richard. The Making of the Atomic Bomb. p.590 (paperback)
Probably the reason why the bomb had to be so substantially assembled right before use had to do with the initiator – a sub-component at the very centre of the bomb, designed to produce a handful of neutrons at the critical moment to initiate fission. At the same time, it was critical that the initator not produce even a single neutron before the bomb was to be used.
In early American bombs, initiators were apparently comprised of the alpha particle emitter Polonium 210 (half life 138.4 days) sandwiched between metal foils to keep it from reacting prematurely with the beryllium metal nearby. When the high explosive shell wrapped around the natural uranium tamper and plutonium core of the implosion bomb detonated, the components of the initiator would mix and react, producing neutrons at the same time as the explosives were producing compression.
Details on initiators are still classified, so we can only speculate on how the implosion primaries in modern bombs function.
The whole issue of deployability is relevant to questions of nuclear proliferation insofar as it is more difficult to make a stable, battlefield-usable bomb than to make a device capable of generating a nuclear explosion. That being said, many of the technical details of bomb manufacture have been made available to states contemplating the development of nuclear weapons. That has partly been the product of clandestine activities like the operation of the A.Q. Khan proliferation network. It has also been the consequence of states being insufficiently cautious when it comes to safeguarding knowledge, materials, and equipment.