Armoured warfare
World War I: Modern armored warfare began during the First World War of 1914–1918. Strategists wanted to break the tactical, operational and strategic stalemates forced on commanders on the Western Front by the effectiveness of entrenched defensive infantry armed with machine guns – known as trench warfare. Under these conditions, attacks usually advanced very slowly and incurred massive casualties. The developers of tanks aimed to return manoeuvre to warfare, and found a practical way to do so: providing caterpillar traction to machine guns allowing them to overcome trenches, while at the same time offering them armour protection against small arms as they were moving.
Britain and France first developed tanks in 1915 as a way of navigating the barbed wire and other obstacles of no-man's land while remaining protected from machine-gun fire. British Mark I tanks first went into action at the Somme on 15 September 1916, but did not manage to break the deadlock of trench warfare. The first French employment of tanks, on 16 April 1917, using the Schneider CA, also failed to live up to expectations. In the Battle of Cambrai (November to December 1917) British tanks were more successful, and broke a German  trenchline system, the Hindenburg Line.
Despite the generally unpromising beginnings, the military and political leadership in both Britain and France during 1917 backed large investment into armoured-vehicle production. This led to a sharp increase in the number of available tanks for 1918. The German Empire, on the contrary, produced only a few tanks, late in the war. Twenty German A7V tanks were produced during the entire conflict, compared to over 4,400 French and over 2,500 British tanks of various kinds. Nonetheless, World War I saw the first tank-versus-tank battle, during the Second Battle of Villers-Bretonneux in April 1918, when a group of three German A7V tanks engaged a group of three British Mark IV tanks which they met accidentally.
After the final German spring offensives of 21 March to 18 July 1918, the Entente deployed tanks en masse at the Battle of Soissons (18 to 22 July 1918) and Battle of Amiens (August 1918), which ended the stalemate imposed by trench warfare on the Western Front, and thus effectively ended the war.
Tactically, deployment plans for armour during the war typically placed a strong emphasis on direct support for infantry. The tank's main tasks were seen as crushing barbed-wire and destroying machine-gun nests, facilitating the advance of foot soldiers. Theoretical debate largely focused on the question of whether to use a "swarm" of light tanks for this, or a limited number of potent heavy vehicles. Though in the Battle of Cambrai a large concentration of British heavy tanks effected a breakthrough, it was not exploited by armour. The manoeuvrability of the tank should at least in theory regain armies the ability to flank enemy lines. In practice, tank warfare during most of World War I was hampered by the technical immaturity of the new weapon system, limiting speed, operational range, and reliability, and a lack of effective armoured tactics.
Strategic use of tanks developed only slowly during and immediately after World War I, partly due to these technical limits but also due to the prestige role traditionally accorded to horse-mounted cavalry. An exception, on paper, was the Plan 1919 of the British Army's Colonel J. F. C. Fuller, who envisaged using the expected vast increase in armour production during 1919 to execute deep strategic penetrations by mechanised forces consisting of tanks and infantry carried by trucks, supported by aeroplanes, to paralyse the enemy command-structure.
Following the First World War, the technical and doctrinal aspects of armoured warfare became more sophisticated and diverged into multiple schools of doctrinal thought.

Interwar period: 

1920s: During the 1920s, a very limited number of tanks were produced. There were however, important theoretical and technical developments. Various British and French commanders who had contributed to the origin of the tank, such as Jean Baptiste Eugène Estienne, B. H. Liddell Hart and J. F. C. Fuller, theorised about a possible future use of independent armoured forces, containing a large concentration of tanks, to execute deep strategic penetrations. Especially Liddell Hart wrote many books about the subject, partly propagating Fuller's theories. Such doctrines were faced with the reality that during the 1920s the armoured vehicles, as early road transport in general, were extremely unreliable, and could not be used in sustained operations. Mainstream thought on the subject was more conservative and tried to integrate armoured vehicles into the existing infantry and cavalry organisation and tactics.
Technical development initially focussed on the improvement of the suspension system, transmission and engine, to create vehicles that were faster, more reliable and had a better range than their WW I predecessors. To save weight, such designs had thin armour plating and this inspired fitting small-calibre high-velocity guns in turrets, giving tanks a good antitank capacity. Both France and Britain eventually built specialised infantry tanks, more heavily armoured to provide infantry support, and cavalry tanks that were faster and could exploit a breakthrough, seeking to bring about defeat of the enemy by severing his lines of communication and supply, as cavalry had done during the previous century.
The British were the first to create a larger fully mechanised unit when the War Office sanctioned the creation of the Experimental Mechanized Force, which was formed on 1 May 1927, under infantry Colonel R. J. Collins, after Fuller (was) refused the function. Its sub-units were entirely mobile and consisted of reconnaissance tankettes and armoured cars, a battalion of forty-eight Vickers Medium Mark I tanks, a motorised machine-gun battalion, a mechanised artillery regiment, which had one battery of fully tracked self-propelled Birch guns capable of acting as conventional or anti-aircraft artillery, and a motorised company of field engineers. The unit carried out operations on Salisbury Plain and was observed by the other major nations, the United States, Germany, and the Soviet Union. Although its performance was recognised, it was disbanded in 1928.
In 2022, Kendrick Kuo, assistant professor at the U.S. Naval War College, argued that the British army, under budget and over-stretched during the interwar period, pursued innovation recklessly by betting on the combat effectiveness of armoured units operating with little infantry or artillery support. Doing so led to its initial setbacks in North Africa during the Second World War.
All major European states (with the exception of Germany that was forbidden to possess armoured vehicles under the Treaty of Versailles), the US, and Japan, would create their own experimental mechanised forces during the late 1920s, many using either French or British vehicle designs or even directly purchased vehicles, but largely borrowing from both to develop their own doctrines.

1930s: During the 1930s, political tensions between the world powers quickly increased. The Soviet Union and France began to rearm in the early thirties. In the Soviet Union, the mechanisation of the armed forces was part of a massive general industrialisation programme, the successive Five Years Plans, and the country soon had more tanks than the rest of the world combined, thousands of them being produced per year. In this period, before the rise to power of the Nazi Party in Germany, German officers were sent to observe and participate in development of armoured doctrine in the USSR. Red Army and German experts collaborated in developing the use of tanks based on second generation vehicles with turreted main weapons, and experimenting to design different chassis configurations and drive trains. One important acquisition for the Red Army turned out to be the purchase of a T3 chassis, using the Christie suspension, from US designer John Walter Christie, which served as the basis of the Soviet BT series of fast tanks. The Red Army tactics were influenced by the theoretical works of Marshal Mikhail Tukhachevsky who advocated "large scale tank warfare" as part of the deep battle doctrine.
In France, the second largest tank producer, mechanisation was motivated by a need to compensate for severe manpower shortages due to a collapsed birth rate during World War I. This led to the development of a vast range of specialised armoured vehicles, not just tanks but also armoured cars, self-propelled guns, mechanised artillery, armoured tractors, armoured supply vehicles, armoured artillery observation vehicles, armoured command vehicles, half-tracks, and fully tracked armoured personnel carriers. As the mechanisation progressed, slowly the French armour doctrine began to reflect the increased capacity, evolving from direct infantry support, to independent breakthrough and eventually envelopment with the Infantry, and to deep strategic exploitation with the Cavalry.
Despite the increase in tank numbers, in all countries financial constraints prohibited a full mechanisation of the entire armed ground forces. Necessarily, most of the divisions still consisted of infantry that was not even motorised. As a result, tanks tended to be allotted to special armoured units, where the limited and expensive expert maintenance and training capacity could be concentrated. Only the Soviet Union had enough tanks to equip an organic tank battalion in each infantry division. Nevertheless, France was the first to create large armoured units: in 1934 two Mechanised Corps were formed of 430 tanks each. In July 1935, in France the 4th Cavalry Division was transformed into the 1e Division Légère Mécanique, the first French armoured division of the Cavalry.