So my question is what mission/targets would call for a 9mm (e.g. Sig P226 Mk 25) over a 45ACP (1911) or vice versa?
I have never been a military operator, I have however worked with and for the SF and CAC for months at times.
My take on it is the MK26 was chosen because of its superior reliability, ability to be silenced, its round count and the fact that even a unsilenced round out of it doesnt sound any different than most any other 9mm. The key being if you fired it unsilenced it doesn’t scream "US military over here".
I would also contend that 45 ACP is well liked and used at times by individual operators for reasons that fit the mission. It is often highly desirable to look like everyone else in a particular area so style of dress and weapons fit the need of the mission.
Also while I’m thinking about it... Despite what at least a few people seem to want to believe. I have never seen a operator, any military operator carry two pistols into combat, that’s all movie type crap, not reality.
As a overall statement pistols in the Army at least are not generally seen as offensive weapons but rather defensive weapons. Officers like them because they are less to tote around and at the same time they are a liability in a real fire fight because quite often you simply arent going to hit the BG at 100 yards from a moving vehicle with a pistol. Also keep in mind some commanders at various times and places give orders that prevent vehicle occupants from engaging the BG, you simply drive through and catch bullets, while a couple of vehicle mounted weapons are your convoys only defense.
Also for those who say I don’t understand why they didn't choose brand "X"... there must be a reason or reasons they keep choosing SIGs and have for a couple of decades... Other armed forces may carry something different that fits there needs and budget. Certainly some other choices would be cheaper but apparently not better... Any given weapon of any brand might be carried to fit a particular mission or operators taste depending on what their portion of the special operations community allows.