Well, we came really close to a global war during the Serbian-Austrian War of 1914. It was a good thing that nations did not hold by their alliances .
The Asian/Pacific War proves how potientially bloody a global war might have been. Over one million dead in combat between the four principle nations and several million civilian casualties.
I have to wonder at this point how the US Army would have developed if involved in such a global grand conflict. The eventual US involvement in the Asian/Pacific War of 1937-1944 was about it for major operations, and only the counter invasion of the Phillipines was larger than a corps. In terms of ground operations the US was smallest player in this war.
Probablly participation in a global or world war before 1940 would have benefited the US Army in experience at operations larger than a light brigade. Army staff work in the various Pacific island battle, the Phillipines, Formosa ect... had a lot of holes. Various reforms and improvements to the officer schools from 1900 gave a solid theoretical foundation, but there was not much practical experience. Unlike the Japanese who had conducted large scale manuvers for decades and then refined their staff skills in several years of combat experience in China. While the French also lacked army scale combat experience their large conscript army allowed at least field exercises for the corps & army staff officers. Colonial operations had their role in refining the French operational staff skills as well.
Both the US and French soldiers suffered at the hands of the Japanese until experience accumulated. As one observer put it "These Americans learn amazingly fast. They are turning the Japanese strengths into liabilities." That is perhaps a exaggeration, and the French under Tassigny showed a great deal of innovation in the invasion of Formosa & their smaller operations along the China coast. Still the Allied Republics soldiers were much better led and capable in 1943-44 vs 1941 or 42.
After the Pacific/Asian War there was some debate in the US as to creating a large reserve and active service army. In 1946 the Army CoS Arnold proposed a active service Army of 500,000 a reserve of 100,000 trained officers for war time call up and a "State Guard" trained to Regular Army standards of 600,000 - 700,000 men & officers. It is clear Congress never would have funded half that but President Taft simply tossed the proposal in the waste basket. Since then the US Army has never grown larger than 250,000 & averaged closer to 200,000, with a reserve of trained officers & NCOs varying between 40,000 & 60,000. The states militias have been so poorly trained & administered it has never been clear what their real or effective strength has been. The moblization during the Fourth Mexican Revolution in 1978 turned out about 150,000 state militia after 180 days of chaos and wasted motion. How capable any of them were at modern combat is highly questionable. There have been efforts at reform in the thirty five years since but the political and social club aspect of the militias inevitablly interfere. Even today some state miltia regiments have nothing resembling a modern field uniform. They turn out in parade dress dating back to the Civil War.
Since the Pacific War the US Army did take the lesson of up to date training to heart. While small the NCO & officer training has been greatly improved and the various small actions since show a reasonable capability. Unfortunatly the small size and lack of a ready reserve of any significance has hampered the US in enforcing its best interests in a number of international crisis through the latter 20th Century. The US navy can show up anywhere we need it in a hurry, but there is no depth or staying power behind in in the Army Ground or Air Forces.
One effect of the small size of the US Army was the USN creating its own army out of its Marine companies. This went back to the base defense battalions & expeditionary battalions of 1910 - 1925. Those were expanded in the 1920s into the inovative combined arms "Expeditionary" Brigades, which proved their worth in the Asian Pacific War. Tho the Taft administration ordered those dissolved post 1946 they were reprieved as essential to the USN bases and operations in the Pacific, and globally, to the present day. The 60,000 man USMC with its three combined arms brigades or air, ground, & support forces (plus a fourth made up of ready reservists) have proved useful & several other nations naval forces have developed similar 'afloat' expeditionary forces.
How the US Army or Navy might have developed given some other larger war during the 20th Century is impossible to say. Perhaps something like the Arnold board proposal would have become accepted as necessary. That propoal projected a large portion of the regular army and ready reserve officers/NCO forming a cadre for some four million recruits. By 1946 standards that was a army of some fifty plus divisions and a similar number of airwings of 200+ aircraft each. A army of that size boggles the mind. I've seen OB for the old European 'reservist' armies of the first third of the 20th Century that in theory could field a even larger force of up to 100 divisions on full mobilization, and those were large robust formations, not the anemic four or five thousand man divisions China fielded against Japan. The European conscripted reservist infantry division of 1910 or 1930 was a massive thing of 20,000 to 25,000 infantry & artillerymen. So in theory the US could have fielded something of that size. Certainly the number of aircraft built for the Asian/Pacifc War & the number of bomber wings deployed was impressive.