I think the theater commander for SWA has banned personal sidearms, or at least that is what others on this board have posted.
It was almost always banned by a CG somewhere on most deployments I went on, many moons ago, but technically, the responsibility lies with the unit commander. Most OPLANS I saw regarding a real deployment had a line in them somewhere that banned personally-owned weapons, which would count as a lawful order.
That being said, in two of the units I was in, personally-owned sidearms were allowed and encouraged by the unit commanders. In one unit, it was limited to personally owned 1911s that could pass for issued ones, and it was considered kind of mandatory for officers to get a tricked-out .45, at least if they were going to share the Battalion Commander's vision. In the other unit, the XO and Company Commander both had FFLs, and would actively sell pistols to their troops. They would sign out one the pistol ranges every Sunday morning, so that their guys could come out and shoot, and if you didn't bring enough ammo, the XO also sold boxes of 9mm and .45 that he reloaded for pretty decent prices.