At the beginning of the First World War, Russia had nine armies, in 1916 – thirteen. Germany had fifteen armies at the end of the war, France had ten, and the United States had one. During the period of foreign military intervention and the civil war of 1918-1920. Soviet Russia deployed sixteen armies. In the Second World War of 1939-1945, there was a further increase in the number of combined arms armies. By the end of the war, Nazi Germany had 17 combined arms armies, Japan – thirteen, the United States – eight armies. The increase in the number of combined arms armies is explained by the increased intensity of the struggle in wars, the expansion of strategic fronts and the increase in the armed forces (the number of divisions, corps) with significantly increased economic opportunities.
Since the 19th century, combined arms armies consisted of 2-5 army corps and 1 cavalry corps (1-2 cavalry divisions). With imperfect controls (lack of telegraph and telephone), an army of such a large composition could only be controlled with its compact combat formation on a narrow front that could be viewed from one command post. By the beginning of the Patriotic War of 1812, the largest of the three Russian armies in composition: the army included 6 infantry corps (12 infantry divisions), 3 cavalry corps, 18 Cossack regiments; In total, it had 149 infantry battalions and 144 squadrons; the total number was 127 thousand people, 558 guns. Russian Russian army was fielded in the Russo-Turkish War of 1877-1878, consisting of 6 corps, which included 186 battalions, 213 squadrons, 774 guns; its number was 303 thousand people, 90 thousand horses. The difficulty of operational management of such a bulky army caused its fragmentation into detachments. According to the deployment plan of 1880, the Russian military command envisaged armies consisting of 11-15 infantry and 3-4 cavalry divisions (177-241 battalions, 96-140 squadrons, 476-622 guns). But the deployment plan of 1890 provided for armies of 17-18 infantry divisions, 10-12 calculated cavalry divisions. According to the 1900 plan, the composition of the Russian armies was envisaged in 9-12 infantry divisions and 4-7 calculated cavalry divisions, but the number of armies as a whole increased.
Due to the significant growth in the composition of the armed forces as a whole and the special conditions of operational management (two separate theaters) in 1900 – for the first time in the history of military art – in Russia, according to the war plan, the creation of higher operational associations – fronts, each comprising two or four armies. The emergence of the concept of the front caused, in turn, the emergence of new concepts: an inseparable army, which is part of a front-line association, and a separate army, which is not part of the front. In other states, higher operational associations similar to the fronts called army groups were created only in the second half of the First World War of 1914-1918.
In the absence of front-line control, it was considered impossible to control a large number of armies, and therefore the latter were created very large. Thus, the Second Russian Army in Manchuria in early February 1905 included 4 army (rifle) corps and 2 large detachments, and in total about 100 thousand people, 120 infantry battalions, 79 squadrons and hundreds, 439 guns. Such a composition of the combined arms armies of the manufacturing period of the war (before the advent of cars, automatic weapons, airplanes, tanks and other types of military equipment characteristic of the machine period of the war) was determined by the fact that their basis was the muscular strength of man and horse (infantry and cavalry). Apart from artillery (which was dominated by calibers no higher than 75-76 mm), there was no other military equipment. Rifle, saber, horse – the main fighting force of the army. The striking force was based on the bayonet, and the maneuverability was based on the mobility of a pedestrian (25-30 km per day).
The typical composition of the Russian combined arms armies of the First World War, deployed in 1914, was: 4 army corps, 3-4 separate second-line infantry divisions, 11-13 infantry divisions in total, 4-5 cavalry divisions, with a total strength of 220-250 thousand people (176-208 infantry battalions, 128-158 squadrons, 360 machine guns, 684-768 guns). Some Russian armies had up to 5 army corps. In the initial period of the war, the Russian armies advanced in a band 80-120 km wide. Such a large width of the front is explained by the linear forms of operational formation of troops and the vastness of the spaces of the Russian-German-Austrian theater of operations. The typical composition of the German combined arms armies in 1914 in the West was 5 army (infantry, reserve) corps, 1 cavalry division or 1 cavalry corps, 1-2 Landwehr brigades. Combined arms armies of 6 army corps, 1-2 cavalry divisions, 276-324 machine guns, 650— 750 guns were deployed in the main direction. The total number of the army is 180—230 thousand people. The German armies in the Western European theater of war deployed for an offensive in a 25-60 km wide strip. Such narrow fronts of the army's actions were explained by the need to allocate siege corps from the army to attack fortresses on the offensive path, as well as the desire of the Germans to have superiority of forces to the full depth of the strategic operation and, in part, the well-known "tightness" of the theater of war.
The management of such large armies in the First World War was facilitated, compared with the 19th century, by the introduction of the telegraph, telephone, radiotelegraph and automobile. The combat power of the combined arms army grew continuously due to the armament with rapid-fire infantry weapons and rapid-fire artillery (late 19th century), and then with machine guns (by the beginning of the First World War), guns of larger calibers (105, 122 and even 150 mm) and mortars (during the war). An increase in operational density, that is, the degree of saturation of the front with troops and firepower, at the beginning of the First World War, with the linear strategy and tactics of that period, was achieved, as a rule, by narrowing the army band, since there were no strategic and operational reserves at the expense of which it would be possible to significantly change the composition of the army at that time. During the positional period, the total width of the offensive front of strong combined arms armies did not usually exceed 30-35 km, and the breakthrough area was 12-25 km, despite the fact that the composition of the combined arms army was brought up to 16-28 infantry divisions (4-5 multi—division corps), up to 1600-2800 medium and higher caliber guns (without mortars) and up to 300-500 combat aircraft. The British and French also reinforced the armies with 300-350 tanks. The narrowing of the front of attack of the combined arms army, despite its immeasurably increased combat power by the end of the First World War, is explained by the increased strength of positional defense, which required a deep multi-layered combat formation to overcome.
During the period of foreign military intervention and the civil war of 1918-1920, there were no corps in the Soviet armed forces, and the armies consisted of 3-6 infantry and cavalry divisions. The states of the division corresponded to the states of the corps. During this period, for the first time in history, the mounted army, the labor army, and the reserve army appeared. The cavalry army is the highest operational cavalry association, which was based on the highest cavalry formations, it was a strong means of operational maneuver and strike in the hands of the front and supreme high command. It included 3-41/2 cavalry divisions and temporarily attached rifle divisions, reaching 18 thousand sabers, 362 machine guns, 52 guns, 5 armored trains, 15 aircraft. The mounted army played a major role in the defeat of foreign interventionists and White Guards. The labor armies consisted of troops liberated due to the liquidation of a number of fronts in 1920; they were used to restore transport and the national economy destroyed by the war. The reserve army united reserve troops, prepared mass replacements for the front, and at the end of the war, along with the labor armies, participated in economic construction. The labor and reserve armies were disbanded with the end of the Civil War.