Which military developed tanks first for use during World War I?

the British this Invention Virtually ended Trench warfare

The British UK was 1st in inventing the tank and in 1st getting them on the battlefield (Sept. 1916) for 1st use in battle of Somme.

The French were 2nd getting some trials/ prototypes out 1917 and real production 1918. 

The Germans were taken by surprise, generally missed out and relied on making anti-tank guns. 

The War ended Nov. 1918.

Porto Tanks were developed by a number of countries at almost the same time.  The first combinations of the three principal components of the tank appeared in the decade before World War One. In 1903, Captain Léon René Levavasseur of the French artillery proposed mounting a field gun in an armored box on tracks. Major William E. Donohue, of the British Army's Mechanical Transport Committee, suggested fixing a gun and armored shield on a British type of track-driven vehicle. The first armored car was produced in Austria in 1904. However, all were restricted to rails or reasonably passable terrain. It was the development of a practical caterpillar track that provided the necessary independent, all-terrain mobility.

In 1911, a Lieutenant Engineer in the Austrian Army, Günther Burstyn, presented to the Austrian and Prussian War Ministries plans for a light, three-man tank with a gun in a revolving turret, the so-called Burstyn-Motorgeschütz. In the same year an Australian civil engineer named Lancelot de Mole submitted a basic design for a tracked, armored vehicle to the British War Office. In Russia, Vasiliy Mendeleev designed a tracked vehicle containing a large naval gun. All of these ideas were rejected and, by 1914, forgotten (although it was officially acknowledged after the war that de Mole's design was at least the equal to the initial British tanks). Various individuals continued to contemplate the use of tracked vehicles for military applications, but by the outbreak of the War no one in a position of responsibility in any army gave much thought to tanks

Wikipedia has the answer, lazybones. I know because I just checked and found the answer in under ten seconds. Try it. It's not exactly rocket science.