A sniper is a marksman who operates alone, in a pair, or with a sniper team, to maintain close visual contact with the enemy and shoot them from concealed positions or distances exceeding their detection capabilities. They generally have specialized training and use high-precision rifles and optics, and often feed information back to their units or military bases.

During the interbellum, most nations dropped their specialized sniper units, notably the Germans. Effectiveness and dangers of snipers once again came to the fore during the Spanish Civil War. The only nation that had specially trained sniper units during the 1930s was the Soviet Union. Soviet snipers were trained in their skills as marksmen, in using the terrain to hide themselves from the enemy and the ability to work alongside regular forces. This made the Soviet sniper training focus more on "normal" combat situations than those of other nations.

Snipers reappeared as important factors on the battlefield from the first campaign of World War II. During Germany's 1940 campaigns, it appeared that lone, well-hidden French and British snipers could halt the German advance for a significant amount of time.