My understanding is they agreed to go to the 9mm since it was NATO policy to use common calibers. That meant 9mm since all the other nations used it.
The Air Force did the original tests which were very thorough. This was done at Eglin AFB in Florida. The Beretta was the only pistol which survived the tests.
The Army being unhappy with the Air Force being given the original tests insisted that they should do the testing. In the second round of tests, only the SIG P226 and Beretta passed all phases. I have read that it was awarded the contract on price as there was basically no difference in the two guns performance. They both had performed superbly.
Some of the special forces types wanted other guns such as the SIG or HK in .45 or the old 1911 and were given permission to purchase them for their own needs.
Now it looks like the 1911a1 will be back for a full department, the Marines. Probably should have just kept it.
“My understanding is they agreed to go to the 9mm since it was NATO policy to use common calibers. That meant 9mm since all the other nations used it.”
Bingo!