When? This evolved over time to what we know in modern baseball today. The first known time the players were instructed to run counterclockwise was in 1839 (page 40, Early Professional Baseball and the Sporting Press, R. Terry Furst, 2014).
One mandate regarding running counterclockwise in the MLB was made after Herman Schaefer stole first from second(1). This is covered under Rule 5.09(b)(10) of the 2016 MLB Rule Book.
Any runner is out when:
- After he has acquired legal possession of a base, he runs the bases in
reverse order for the purpose of confusing the defense or making a
travesty of the game. The umpire shall immediately call “Time” and
declare the runner out;
Why? This article dives into factors that can be summarized by stating that the game of baseball developed into what it is today because the general population is predominately right-handed. This, as a result, influenced specializing skills such as switch hitting and left-handed batting rather than by default (ie, clockwise baserunning, a mirror image of what we know today).
A few quotes from said article:
Clockwise baserunning, therefore, would necessitate that most fielders
would have to turn before they threw the ball ...
Since a majority of the population would find it easier to throw
without first turning, and to swing the bat right-handed,
counterclockwise baserunning gradually prevailed.
Now the environment of the game can influence skills that can be
learned. Professional baseball evolved with batters standing on either
side of the pitcher-catcher axis. ... The
left-handed batter stands closer to first base, and this provides his
team with an offensive advantage. He can both reach first more
quickly, and block the catcher's view of the runner on first.
Baseball developed in a manner favoring the very specialized skill of
right-handed throwing, and the less-specialized technique of
left-handed hitting. Most ball-players at all positions throw
right-handed; many bat left-handed, however, and there have been very
few hitters indeed who bat right and throw left. Because of the
necessity for speed in throwing, there have been no regular, longtime
southpaws at C, 2B, SS, or 3B in this century.