I think this just barely qualifies under the 20 year rule. It appears that the U.S. policy was instituted in 1992 as a result of DoD Directive 5210.56, signed by Donald J. Atwood, Deputy Secretary of Defense, under George H.W. Bush, (Source: The Blaze.com) but there is widespread conflicting information about this online.
I was curious about the actual motivation for this directive and also about the historical precedent. (Not wearing swords in the company of the king type of thing).
I don't know about the US, but weapons in the UK military are either in use or being stored in an armoury on base, and it's been that way since at least the 1850s.
There were a series of IRA attempts to break into these armouries during the 1960s.
Sources: Redcoat by Richard Holmes and The Making of the British Army by Alan Mallinson.