The
densepack is a strategy for basing
ICBMs (specifically the
MX Peacekeeper) so as to maximize their
survivability in the event of a
nuclear attack on their
silos. It is brought to you by
the Reagan Administration, some overeager thinktankers, and the letter $$$.
So, you have these incredibly expensive, incredibly accurate missiles. You want to make sure that they (or at least, as many of them as possible) survive a Soviet first strike on their silos. The first step is to harden the silos. This was done; the MX silos were hardened to the point (at least, so the Air Force claimed) that any nuclear strike would have to be a ground strike and the resultant crater would have to impinge on the silo for the MX to be destroyed or disabled.
This is debatable. However, the silos were extremely hard.
In any case, the next step is to do whatever you can to make sure that the incoming warheads don't reach their targets. The ABM Treaty that much of the cadre of idiots currently populating the U.S. Government are trying to throw away by reviving SDI (another Reagan legacy) made sure that you couldn't do anything as insane as have nuclear ABMs near your silos. So there needed to be another way.
Enter densepack. This entailed bunching the MX silos fairly close together in a line. The line would run approximately north to south - along the ground track of any probable incoming ICBM. The logic ran like this: If the silos are attacked, any ground strike on one or more of them would throw an enormous amount of debris into the air. Successive incoming warheads would be forced to fall through these clouds of debris, and would be increasingly likely with each detonation to hit some of it, neutralizing them.
IMHO, this is complete nonsense. There are easy ways to defeat densepack even assuming the silos were/are as hard as the Air Force claims. In any case, it didn't get done, which either showed a remarkable outbreak of smarts on the part of the U.S. military establishment, or a fortuitous happenstance involving budgets and domestic politics.
Other deployment strategies include(d):
...and, no doubt, many more that never saw the light of day.