I was told by a veteran that it was not until the Korean war that American soldiers began to balloon their pants. Prior to that it was only paratroopers. Can this be validated?
“Ballooning” is somewhat nebulous… does this refer to “blousing” or does it refer to, as they say, “stuffing” one's trousers inside the boot top?
In WW2 the US Army started out with a standard ankle high field boot, see http://www.usww2uniforms.com/9-6F.html . Over this was worn canvas leggings which also covered the trousers, see http://www.usww2uniforms.com/figures_army.html. The “ballooning” effect could be had by allowing some slack were the trousers entered the legging. This effect also occurred in general wear as when someone having just laced up his leggings were to squat, causing the trousers to slightly pull up/out (actually a good idea as it permits an additional modicum of freedom of movement). Wearing of leggings in the field was prescribed for all personnel, except those in animal mounted, animal drawn or pack mounted, in the 1941 uniform regulations (see paragraph 29c) http://www.usww2uniforms.com/bib_AR600-40_28aug41.html
In 1943, the Army started issue of a combat boot, see http://www.usww2uniforms.com/BQD_114.html. Note that this boot is essentially an extension of the earlier field boot with an added buckled leather upper reaching up the calf. One also tucked one's trousers into these boots, and, if smart, also did the squat thing. Thus, the trouser legs were in the tops of the boots and slightly bagged where they met the boot top. Whether or not this can be called “ballooning” is a matter of personal opinion. To me, yes, it could be so termed.
Paratroopers were issued a laced boot that provided support over the ankle and up the calf. Originally the trousers were also to be inside the boot. At some point, and here I am unsure if this is a WW2 timeframe or somewhat later in the program (though I lean towards a post-war phenomenon), paratroops took to “blousing” their trousers on the outside of the boot. This entails use of an elastic device, rubber bands, elastic bands, condoms (yes, I've seen that, but my take was that someone was just trying to be a comedian), and such. (In modern times there are these nifty little devices, "blousers," to accomplish blousing, shown here as sold by the Aggies https://shop.corpsofcadets.org/products/boot-blousers-green). In the lore, the paratroopers came to espouse the belief that they were the sole members of the US Army permitted, note, “permitted,” not “authorized by regulation,” to blouse their trousers outside their boots (though the expression is to “blouse boots” not “blouse pants” or “blouse trousers”) and took grave offense at any non-paratrooper types blousing their boots.
From an appearance standpoint, stuffing one's trousers into either leggings or boots creates a “ballooning” effect and blousing simply exaggerates it. Today, blousing is fairly common, but not to say that troops do not stuff their trousers into their boot tops as well. Sometimes whichever practice is found could be at the preference of the commander. Modern US field uniform trousers have a draw string or draw ribbon inside the bottom hem that allows the wearer to tie off the bottom of the trousers over the boots.
I have seen this gone to extremes in some garrison units. One puts on his trousers, situates the bottom hem of the leg where he wants them, folds the trouser bottoms to form a neat V in the back and duct tapes or uses blousers to hold the trousers legs in place. Then put on and lace up the boots over the bottoms. Now, before pulling up the trousers, place a chain or some slightly weighted device into the trousers where they meet the boot and then pull up and secure the trousers. This creates a balloon effect that stays in place without blousing on the outside. Looks really spiffy in a garrison environment, but totally impractical for wear in the field.