I voted for the 1934 Beretta .380ACP (Italy)
.
My reason; I am already armed with an M1 Garand 30-06 rifle, and the Beretta weighs only about half what the Colt .45 weighs. .380 ammo is lighter than .45 ammo, so I can carry extra clips/ammo. My rifle is for 100%% of my battle needs. The pistol is for close range, last ditch, hand to hand combat range fighting while I try to reach my rifle. If my rifle breaks, then I'll be finding another US rifle (or a pump shotgun) ASAP.
If I was limited to having ONLY a sidearm, with no rifle, then of course the .45 would be my top choice. But where I'm already armed with an M1 rifle, the sidearm is for when I'm at the latrine, in my bedroll or dealing with a POW up close, during a search, etc. The 95 grain .380 hardball in a vital zone such as the head or heart was (and still is) lethal, and the gun/ammo weigh much less than the Colt .45. The Beretta is a rugged well built weapon, and the open slide design eliminates jams, a major concern in dirty, dusty, muddy battle zones. And, it is a small pistol and will easily tuck in my waistband or a pocket, no need for a holster on my belt. I want as much room as possible for extra Garand .30-06 clips and ammo!
I have carried a Colt 1991A1 .45 as a sidearm a few times when bowhunting deer or as a companion to my .22 rifle when hunting jackrabbits in deep snow. I have also used the same Colt 1991A1 and a Springfield 1911A1 as a CC gun. These are HEAVY guns! The Colt .45ACP is a great weapon and caliber... however, it seems to get heavier every mile it is carried...
So I would either trade US smokes for, or "liberate" a Beretta .380 and a few extra magazines/ammo, and not think twice, and then get back to cleaning my M1!!!!!!!!!
Now I'm stating the above as a "WW2 foot soldier", who often has to cover 10-20 miles a day and carry lots of gear. If I was in a Tank and didn't have to lug gear around; A Tommy gun in .45 w/lots of clips, and the Colt .45 1911A1.