[parsifal]

Didn't the old 45 work pretty well? If so, why did US switch? parsy.

[LibKill]

Didn't the old 45 work pretty well?

Yes, very well indeed.

If so, why did US switch?

Cold War politics. 9mm is the standard NATO pistol round. And, by the time of the switch, the .45s in service were all 40+ years old and kind of worn out.