thumb|Jakob Meckel.Klemens Wilhelm Jacob Meckel (March 28 1842 - July 5 1905) was a general in the Prussian army and foreign advisor to the government of Meiji period Japan.

Meckel was born in Cologne, Rhine Province. He graduated from the Prussian Army Staff College in 1867.

In Japan

After the government of Meiji period Japan decided to model the Imperial Japanese Army after the Prussian army, following the German victory over the French in the Franco-Prussian War, Meckel (with the rank of major at the time) was invited to Japan as a professor at the Army Staff College and as an advisor to the Japanese General Staff. In response to a Japanese request, Prussian Chief of Staff Helmuth von Moltke selected Meckel. He worked closely with future Prime Ministers General Katsura Taro and General Yamagata Aritomo, and with army strategist General Kawakami Soroku. Meckel made numerous recommendations which were implemented, including reorganization of the command structure of the army into divisions and regiments, thus increasing mobility, strengthening the army logistics and transportation structure, with the major army bases connected by railways, establishing artillery and engineering regiments as independent commands, and revising the universal conscription system to abolish virtually all exceptions. A bust of Meckel was sited in front of the Japanese Army Staff College from 1909 through 1945.

Although his period in Japan (1885-1888) was relatively short, Meckel had a tremendous impact on the development of the Japanese military. He is credited with having introduced Clauswitz's military theories and the Prussian concept of war games (kriegspiel) in a process of refining tactics. By training some sixty of the highest-ranking Japanese officers of the time in tactics, strategy and organization, he was able to replace the previous influences of the French advisors with his own philosophies. Meckel especially reinforced Hermann Roesler's ideal of subservience to the Emperor by teaching his pupils that Prussian military success was a consequence of the officer class's unswerving loyalty to their sovereign Emperor, as expressly codified in Articles XI-XIII of the Meiji Constitution.

On the General Staff

On his return to Germany, Meckel was promoted to major general, and placed in command of German forces in the Rhine area. He was named editor of the 2nd and 3rd editions of Schellendorf's Duties of the General Staff ('Der Dienst des Generalstabes).

Meckel's reforms are credited with Japan's overwhelming victory over China in the First Sino-Japanese War of 1894-1895.

Meckel's tactical over-reliance on the use of infantry in offensive campaigns was considered to have contributed to the large number of Japanese casualties in the subsequent Russo-Japanese War of 1904-1905.