Dealing with "General Winter" has always been an issue with fatal consequences if not accomplished well. Before modern means of weather adaptation it was army convention to cease fighting once winter fell...this was part of the convention-breaking that made a difference in favor of the Patriot cause in the American Revolution. In the musket age (after discontinuation of wearing old-style metal armor that was ineffective against firearms but before the general use of repeating arms) standard uniforms were suited for temperate conditions while cold weather involved the adding of components such as overcoats and scarves.
This remained the general case through the world wars but that era saw the gradual introduction of cold weather layers as standard uniform components rather than just situational supplements. For years the British Empire had already been adapting as their bush wars brought them all over the world, but for the US it was WWII that had them fighting in such a huge variety of climates it drove new uniform and gear developments across the board, from trading in wool blends for lightweight cottons in the humid tropics to field jackets and pile caps for the European winter. Elite mountain troops were the first to make use of issue parkas, arctic rucksacks and sleeping bags when the respective equivalents for regular troops was additional layering (often with components sent from home rather than issued), getting creative with the standard field-cargo pack ensembles and being issued extra wool blankets.
Then came Korea, fought at the very beginning of the life of NATO thus the standardization agreements that would turn the militaries of the First World into the essential example of modern fighting forces was still years away. This meant the war was fought more like WWII than any other Cold War-era conflict but also that developments which started during WWII found new pertinence and would finish in the 1950s. In the US Army this included new across-the-board uniform standards on a new dichotomy (garrison vs. working) covered a couple posts back, adoption of the M-1956 load carrying system which replaced all predecessors, and for the first time a materials and layering cold weather ensemble standard for issue to any and all troops: the M-1951.
Today standard US Army issue is the Enhanced Cold Weather Clothing System Generation III; starting in ECWCS Gen II the Air Force and Marines took their own path but materials and concept are still more common than different. While previous ensembles played around with numbered layers and the crafting of standard setups for anticipated conditions this is what really set it in writing. The chart form is copied here:
The rest of this post will explore the development of each of these 7 layers and how they were intended to be used. Where pertinent components of other branches will be touched upon though this does not really occur until the 21st century: before all branches drew from the same equipment pool much as was the case with ALICE web gear. The big caveat is when it came to combat effectiveness following by-the-book standards always took a second seat to troops keeping warm and alert, so a complete unit suiting up with no-more-no-less than specified in the charts is probably not that common; individual tolerances differ and the primary purpose is to get the job done.
Starting Notes
Sources include the Clothing & Individual Equipment Hub (https://ciehub.info/clothing/CW/?sort=path) with historical overviews on Olive Drab (https://olive-drab.com/od_military_gear.php). This review is my own perspective and may not be 100% factual in regard to design philosophy. It is not all-encompassing as it covers only those complete systems made for general issue... M-1951: the first complete and unified cold weather ensemble for regular troops
M-65: same form and function as M-1951 made with updated materials (both were interchangeable)
ECWCS I: the first Gore-Tex shell system, carried over a few components from M-65
ECWCS II: partial upgrade of outer shell and lighter inner layer option, otherwise same as Gen I
LEP: Polartec fleece layering upgrade that was originally special issue but later made general availability
MCWCS: USMC successor to ECWCS II with some components adopted by the Air Force, still issue
ECWCS III: culmination of ECWCS II with LEP-type inner layers, still Army issue
Protective Combat Uniform (PCU), the successor to Lightweight Environmental Protection (LEP) will not be elaborated due to its continued special issue status, however it is extremely similar to ECWCS III (with a bit of MCWCS flavoring) and is favored by the Navy. The modern 7-layer ECWCS/PCU convention will be followed as closely as possible but since 20th century-vintage systems were more like 5 layers there is going to be some combinations and a little idea stretching.
Layer 1
Before the liabilities of doing so were understood it was not uncommon for base layers to be cotton for many armies; the material was comfortable, non-inhibiting and relatively lightweight, but upon sweating it became a problem rather than asset in the cold. Later M-1951 and M-65 inner layers were wool blend and stores of these remained issue until the end of the 20th century. In any case there was only a single base layer, with multiple choice base layers a concept implemented several systems later.
One of the major upgrades of ECWCS was a fully-synthetic base layer eliminating the moisture retention issue of cotton and the skin comfort issue of wool (though at the expense of fire protection). ECWCS II introduced a lightweight option that could be worn by itself in milder conditions or combined. LEP used Polartec material which carried into MCWCS and ECWCS III, all as a lightweight first layer rather than the heavier polypropylene developed as the all-purpose base layer of ECWCS I.
Layer 2
With ECWCS II and LEP introducing lightweight underwear the medium-weight moisture-wicking layer either got ditched altogether in mild cold, used same as before or both layers combined to augment for more extreme cold. For ECWCS II the same polypro remained, supplemented and not replaced by the birth of its littler sibling; LEP and MCWCS introduced new midweight-specific components designed to work in tandem with the lightweight layer when needed. It so happens around this same period of time there was also introduced the Modular Sleep System which followed a very similar concept: lightweight sleeping bag for mild, a heavier-weight one for intermediate, and both could be combined into one for extreme cold.
Layer 3
Pre-ECWCS this was the official OG-108 Cold Weather Uniform specified for wear in AR 670-1 all the way through its 2005 edition. Being wool it was unsuitable to wear with anything underneath other than full-coverage base layers but so long as the uniform stayed clean it worked well for that niche purpose. It was succeeded as a cold weather layer (but not as a uniform) by the [in]famous "bear suit" which did its job so well at least one anecdote online refers to its wear causing a heat casualty in the arctic. Standards diverged widely for this layer afterwards: ECWCS II kept the same use but replaced it with a lighter and more manageable Polartec fleece ensemble, LEP designated this layer for only the bib overall while the opposite path was taken with ECWCS III by making this layer only the fleece jacket, and MCWCS had no official equivalent at all since the combination of layers 1 and 2 was supposed to accomplish the same.
Layer 4
While the 3rd layer took a turn at being suitable as an outer layer for more tolerable conditions, layer 4 would return to the concept of intending to be worn as a layer underneath only. I will make a stretch to keep thing consistent here as officially for ECWCS I this was supposed to include repurposed M-65 field trousers, but instead I choose to treat those per their original design as an outer layer. With that set this makes the layer strictly the field jacket and trouser liners all the way through ECWCS II: mohair in M-1951 with nylon-polyester for M-65 on (same build and material as the poncho liner). For all systems following ECWCS II this is a fleece or synthetic jacket that CAN be worn as an outer layer conditions permitting as some types (MCWCS and ECWCS III) are printed with uniform camouflage patterns for this purpose.
Layer 5
When a cold weather ensemble is worn this is typically the layer that shows it. In WWII the field jacket became a regular uniform item to keep and throw on for inclement weather, a practice continuing today with Gore-Tex outer shell jackets. Field trousers were made of the same material and in conjunction with their liners were very effective insulators...too effective if overused hence proper ventilation and layering practices. M-1951 jackets and trousers were weatherized cotton sateen, changed in M-65 with minor cut improvements to nylon-cotton. M-65 shells were retained well into the ECWCS era, now printed in M81 Woodland camouflage: officially the trousers as an ECWCS option and jacket as a uniform item (as such the jacket would later also be printed in 3-color desert and Army UCP patterns), unofficially both were new outer layer for those units not yet endowed with sufficient Gore-Tex for general issue. For LEP this is a fleece wind jacket and its final layer (the system is meant primarily to be inner layers used with Gore-Tex shells of regular ECWCS) which ECWCS III went a similar route as a soft wind-resistant shell (basically a stronger variant of layer 4); the Marines took a different path that is elaborated below.
Layer 6
Pre-ECWCS this is strictly the outer layer for extreme cold: parka with liner, worn in addition to the field jacket with liner for maximum insulation, and for M-1951 there are arctic trousers with its own liners also worn over the regular field trousers for best effect (the trouser system changed slightly in M-65, detailed below). For all others systems this layer is the Gore-Tex outer shell, ECWCS I the baseline standard and still easiest to find on the military surplus market. What I also count in this category, even though it was usually addressed separately for special use, is the only nylon-cotton fishtail parka-trousers combination printed in a US issue camouflage: the Night Desert ensemble.
At the eve of the Global War on Terror contemplations were already being made for ECWCS II including the build of the outer shells. In the end it wasn't adopted widely because further developments resulted in deployment of successor systems soon after; it took me years to get a complete set of USMC ECWCS II in M81 Woodland and I'm unsure if the Army variant in UCP is any rarer because at first glance it tends indistinguishable from UCP-printed ECWCS III. The succeeding Marine variant is called the All-Purpose Environmental Clothing System (APECS) which got picked up by the Air Force who had it printed in their brief fling with Digital Tiger Stripe.
Note for fellow collectors: so far as I've been able to ascertain official issue ECWCS I shells are available only in M81 Woodland and 3-color desert while ECWCS II shells are available only in M81 Woodland and Army Universal Camouflage; commercial clones of course vary widely, with special mention made for the reversible camouflage variants (including Night Desert of all patterns) by Adventure Tech which were not issue but testimony exists that they were unofficially acquired for field use anyway.
Layer 7
In the 20th century this was the snow camouflage layer with one key exception: M-65 eschewed arctic trousers altogether instead offering a white-colored liner for the overwhite trousers. In the 21st century this became the extreme cold outer layer with snow camouflage components optional. From M-1951 through ECWCS I the same overwhites were used (which were NOT insulation) changing only a little in fabric until ECWCS II introduced a new type in a different self-containing cut. At least until recently the latter remained Army issue while Marines adopted a new oversuit with a digitized arctic camouflage to go with the rest of the MARPAT system. The actual insulation layer for MCWCS and ECWCS III is solid-colored, apparently because anticipated conditions of their use would have little color variation in the terrain anyway.
Personal note: while I possess ECWCS-era and older samples of all other layers I don't for this one since living in Texas I'm not anticipating the arctic conditions requiring such.
Other Items
Socks, boots and gloves have varied in builds and quality improvements but in general they all function the same and any would be effective for the job if used right (though I doubt I'll ever need trigger finger mittens in Texas). Headwear is more diverse and a fascinating study of its own starting with the M-1951 binary choice of the regular issue cap with ear flaps (shelved during the Vietnam era and revived with the BDU now printed in matching camouflage) or the older WWII-style pile cap that with development of M-65 evolved into the helmet liner. Detachable fur ruff hoods, sized to fit over helmets, could interchange among the field jackets and parkas of both M-1951 and M-65, though all were only olive drab so it would do little for appearances on a camouflage field jacket (the only non-ECWCS camouflage hood I found is designed for the fire-resistant aircrew jacket and attaches differently).
Closing
Most of these components are still fairly easy to find in good condition, and all are great at what they do provided they get used correctly. Principles of cold weather protection is beyond the scope of this post, but one big point to make is that even these old systems if still in good shape will perform and they are made in the USA, build to last. About 20 months ago here in Texas we got a rare extreme winter spell, a reminder that no premium can be set on adequate preparation no matter where you are (please pray for post-Ian Florida while we're on the subject).