The idea of an armored fighting vehicle did not spring full blown as a weapon for World War One. Roman chariots, the knights of England and France, the war wagons of the US Civil War were all fighting systems designed to provide battlefield mobility and armored protection. It took the coming together of a number of things in the early 1900s to produce the fighting vehicle system we know today as the tank. The fossil fuel internal combustion engine had been developed to a reasonable degree and could provide the power source for a fighting vehicle. Endless metal belt running gear (track) had been developed for earth moving vehicles (by the Hornsby Company of Grantham, England and the Holt Tractor Company of Stockton, California) and could provide the cross country mobility that wheeled vehicles lacked. Recoil mechanisms for large caliber weapons allowed their use without heavy, fixed firing platforms. And a war in Europe had turned into a stalemate with miles of trenches, protected by barbed wire and rapid firing machine-guns, facing each other across terrain that was mostly rubble and mud.

It was the British, with their well developed industrial capacity, that first put the parts together and actually building a fighting vehicle. Concept development credit is given to LTC E. D. Swinton and design credit goes to LT W. G. Wilson and Sir William Tritton with the first "roll out" on 12 JAN 1916.Credit is given to Sir Winston Churchill, then Lord of the Admiralty, for pushing the concept through the military bureaucracy and into the field. He is also said to have originated the cover story of water carrying vehicles for the forces fighting in Mesopotamia and therefore "Tank".
This first model was named Little Willie

                    and was soon followed by the Whippet.
Based on the experience gained from the Whippet and Little Willie the Mark series went into large scale production. Seven different models in the Mark series were build in some number. The Mark I, easily recognized by its tail steering wheel, the Mark II with machine guns in the side pods instead of the 6 pounder cannons found on the Mark I, the Mark III with thicker armor plate, the Mark IV with retractable gun pods and a top mounted beam used to facilitate crossing wide trenches, the Mark V with a bigger engine and an improved transmission, the Mark VIII built in the USA with a Liberty aircraft engine and much larger than its predecessors (43.5 tons-crew of 5) and the Mark IX built as a troop or cargo carrier. The most successful and the one built in the largest quantity was Mark V. It had a top speed of about 4 mph, could climb a vertical wall of 5 feet and span a trench 8 feet wide, carried a crew of 8 and was armed with two 6 pounder cannon and two Vickers machine guns. The appearance of the tank on the battlefield came as a surprise to the German army. The first tank assault occurred on 15 SEPT 1916 near the SOMME RIVER. 50 tanks were assigned to the operation, 32 reached the assembly area and 9 reached their objective. The Germans were not impressed. Their first large scale employment came at the battle of CAMBRAI in NOV 1917.   316 tanks took to the battle field and 179 were lost. Losses were to enemy action (65), mechanical failure (71) and to the mud (43).

While the battles were not the success hoped for, it clearly showed the potential of the tank. German, French and American designers quickly developed their own version of the tank and soon thereafter fielded them in France. The German tanks included the A7V which mounted a 57mm main gun, 6 machine guns and required a crew of 18. Only about 20 of these monster tanks were built before the end of the war.

The American tank was smaller in an attempt to develop higher speeds and mounted its main gun firing through a port in the front of the vehicle. The narrow tracks hindered its ability to traverse muddy terrain. In an attempt to reduce weight the armor plate was not thick enough to give much protection from enemy fire. For all that it was also under-powered and in most respects not a good fighting machine.

The French fielded the heavy St. Chamond and then the French industrial giant RENAULT designed and built a tank, the FT-17, noted for its use of a top mounted turret that could traverse left and right. It mounted a 37mm rapid fire gun and required a crew of 2.

This design was clearly superior to all other light tanks and the Americans devoted their energy to mass producing US M-1918s (the French FT-17 with steel road wheels rather than wood as on the French version and other minor changes) under license from RENAULT. Working to government contract, FORD MOTOR COMPANY established a new assembly line just to build these tanks.
Tactics of employment also developed at a rapid rate with different ideas and theories abounding. From semi-stationary pill boxes guarding roads, to moving heavy weapon support for Infantry, to mass cavalry style charges; each level of command had a different idea.