POMCUS is an acronym which stands for Prepositioned Overseas Materiel Configured to Unit Sets. These were/are American equipment and supply dumps in continental Europe designed to allow NATO to reconstitute a European ground force presence as quickly as possible. This was done because it would take unacceptably long to prepare, transship, unload and organize U.S. military units (divisions, typically) in Europe, and because World War I and II had shown the vulnerability of the SLOCs. In the event of war, American servicemen and women serving in units in the CONUS would get on military transports or jetliners of the CRAF and fly to various staging points in Europe, where they would be met by warehouses full of equipment which precisely matched what they'd left behind, from main battle tanks to bullets to beans. They would then 'mate up' with this equipment, and voila, you'd have a fresh American battalion or division moved across the Atlantic in days rather than weeks or months, with no chance of the ship carrying your tanks being sunk midway by marauding Soviet attack submarines.

This practice further demonstrates the U.S. tendency to utilize materiel to excess rather than use additional personnel; instead of maintaining more numerous and lightly-armed units in Europe, the U.S. chose to simply buy two of everything for several divisions - not a small price tag when you consider that the average American armor division's equipment costs on the order of $5 billion (with a b). In addition, entire staffs were employed simply maintaining these vehicles and supplies in air-conditioned warehouses against the day they might be needed.

Militarily, of course, if you can afford this method, it's quite effective. The Gulf War was fought in part by American armor units moving equipment to the Iraqi theater via rail from their bases in Germany; in recent times, the POMCUS sets have been depleted and not replaced, or decommissioned as the U.S. force structure changed.

Update 2015:

The U.S. is publicly discussing prepositioning heavy military equipment in Poland and/or the Baltic states in response to Russia's proxy (and probably actual) invasion of Ukraine. Plus ça changeā€¦ Don't know if it would be actually called POMCUS - if it was just certain equipment, not brigade or other unit-sized sets, the term wouldn't apply.