Following the US nuclear attacks on Hiroshima and Nagasaki, it became apparent that nuclear weapons wield unspeakable power. However, it was also apparent that nuclear devices had a remarkable potential for miniaturization, so much so that a nuclear bomb ended up as a charge that could fit into a backpack.
The initial attempt to make a smaller bomb turned out to be remarkably successful: the smallest devices of atomic annihilation could fit into field artillery or even a recoilless rifle, a precursor to an RPG. There was one problem, however: its blast radius was much larger than the projectile's range. Firing this kind of ordnance would have effectively annihilated all of the enemy, along with all of your own.
During the Vietnam War, the US military came up with a different idea. What if one made a slightly bigger bomb that can fit into a backpack? A soldier could carry one deep behind enemy lines, and then use it to destroy sensitive enemy facilities. Well, the military did exactly that, with one catch: the soldier was not supposed to evacuate before detonation.