The Russian Army had a deception school, active in 1904, disbanded in 1929. Meanwhile, military deception was developed as a military doctrine in the 1920s. The 1924 Soviet directive for higher commands stated that operational deception had to be "based upon the principles of activity, naturalness, diversity, and continuity and includes secrecy, imitation, demonstrative actions, and disinformation." The 1929 "Field Regulations of the Red Army" stated that "surprise has a stunning effect on the enemy. For this reason all troop operations must be accomplished with the greatest concealment and speed." Concealment was to be attained by confusing the enemy with movements, camouflage and use of terrain, speed, use of night and fog, and secrecy. "Thus 'in Soviet military art during the 1920s the theory of operational "maskirovka" was developed as one of the most important means of achieving surprise in operations.'" The 1935 "Instructions on Deep Battle" and then the 1936 "Field Regulations" place increasing stress on battlefield deception. The "Instructions" define the methods of achieving surprise as air superiority; making forces mobile and manoeuvrable; concealing concentration of forces; keeping fire preparations secret; misleading the enemy; screening with smoke and technical deception; and using the cover of darkness. In the 1939 Russian invasion of Finland, white winter camouflage was worn by Soviet troops.