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. 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