The zenith of the British Army's combined arms during the First World War was the Battle of Amiens in August 1918. Tanks were used in vast numbers, integration with aircraft and artillery was seamless, and even horsed cavalry made an appearance.

Organised behind an impenetrable deception plan that was later studied as part of the preparations for D-Day, the Battle of Amiens was opened by a stunning bombardment of 2,070 guns at 04:20 on 8 August. 552 heavy tanks (the archetypal rhomboid tanks) led the attack with the infantry, followed by 72 Medium Mark A Whippet tanks for the exploitation phase. One of these Whippets, Musical Box, roamed around behind German lines for nine hours before finally being disabled. The Australian Corps achieved a six-mile advance in one day; the Canadians Corps on the right, eight miles. 504 of the Germans' 530 guns had been identified before the attack and were rapidly neutralised by counter-battery fire. Finally, all-arms groups of horseman, composed of both traditional cavalry and mounted infantry, rode through gaps in the German lines and wrecked havoc, capturing Harbonnières and the line of Caix-Beaucourt. On that day alone the British took 9,000 casualties for 27,000 German, including 15,000 prisoners and 400 German guns. Ludendorff famously named 8 August “the black day of the German Army.”

Amiens and the battles that led up to it like Flers-Courcelette (the very first commitment of the tank on the Somme in 1916) and Cambrai (where the Tank Corps made spectacular initial gains but suffered terribly from a devastating counterattack) informed the British Army's reforms in the 1920s, where the overriding concern was to avoid fighting an attritional war again. In this period, the tank's leading evangelist was Colonel J.F.C. Fuller, lately Chief of Staff of the Tank Corps (and unfortunately, later an unreconstructed fascist). Fuller had helped plan the attack at Cambrai and the battles of autumn 1918, and argued for an all-tank force that would operate entirely independently, breaching the enemy lines to slice through his lines of communication and destroy his headquarters. In Fuller's concept, the infantry would only be needed to guard tank harbours during pauses in the offensive at night. An opposing theorist was Captain B.H. Liddell Hart, who argued that tanks could only succeed if all the combat and supporting arms were mechanised. A tank force that needed to, for example, make an opposed river crossing would be effectively helpless without engineers, mobile bridges, and infantry to secure and guard both banks. Another reformer, Giffard Le Quesne Martel (who in 1940 would lead the counterattack at Arras that put quite a scare up Erwin Rommel before it was contained) went as far to build a prototype tankette in his own garage to demonstrate his theories, advocating that tanks should be subordinated to the infantry.

From 1927 to 1929, the British Army conducted a series of exercises on Salisbury Plain with the Experimental Mechanized Force (EMF), a brigade-sized formation that prompted the establishment of a permanent Tank Brigade in 1931. The EMF combined tanks with armoured cars for reconnaissance, motorised infantry, lorry-bound Royal Engineers, towed and self-propelled Royal Artillery, and three RAF squadrons. The Tank Brigade, however, was formed of four tank battalions and lacked attached infantry and artillery. Liddell Hart excoriated the Tank Brigade's performance in 1934 exercises, condemning them as unrealistic in that the obstacles it had faced could be easily bypassed, thus denying the need for engineers or infantry to protect them.

In the dire economic conditions of the 1930s, there was little interest in funding a brand-new combined arms force, and the British Army would enter the Second World War with armoured doctrine informed by the experience of Amiens, and confirmed by unrealistic exercises between the wars. British armoured formations for much of the war were over-tanked and under-supported: Tank Brigades, equipped with infantry tanks, were expected to cooperate with the infantry to break into the enemy defences. The infantry tank was informed by the experience of the break-in at Amiens, and was typically heavily-armoured, but slow and often under-armed (the Matilda I was armed with a single heavy machine gun). Armoured Brigades, equipped with cruiser tanks, would be committed to exploit the breach. The cruiser tank, like the Whippet, was lightly-armoured and fast, and were typically armed with anti-tank guns in the expectation that they would engage counterattacking enemy armour. However, their guns were often issued with only solid anti-tank shot and lacked a high-explosive shell, which left them critically vulnerable to lines of anti-tank guns set up in defence. The Armoured Brigades were also desperately under supported: the 1940 establishment for an Armoured Brigade had only one mechanised infantry battalion for three battalion-sized armoured regiments. During the North African campaign, armoured regiments (often ex-horsed regiments, fulfilling the worst traditions of the British cavalry) would routinely outrace their meagre infantry support and impale themselves on lines of German anti-tank guns. It was not until 1944 that British armoured divisions achieved a proper balance between armoured regiments and infantry battalions, but these were still split between the Armoured Brigade and the Mechanised Infantry Brigade. It was not until after D-Day that commanders stopped thinking in terms of these being separate brigades and formed combined arms brigade groups of two teams formed of one armoured regiment and one infantry battalion.