Prinz's chess program
Dietrich Prinz's program on the Ferranti Mark 1 solved mate-in-two problems; no attempt was made to have it play a game.
Why it matters
The first chess problem solved by a computer showed both the possibility and the limit: the search was exhaustive, and the machine was too slow to play.
To have something working quickly, the rules were cut down: no castling, no double pawn moves, no en passant or promotion, and stalemate counted as mate. The program did not discard unlikely moves; it tried every move until it found the solution. The simple problem in the report took 15 minutes, about 450 moves at roughly two seconds each, and Prinz wrote that a solution could take several hours. The report called the machine "disappointingly slow" at chess and saw a game against a human as far off. It is Shannon's type A strategy in its purest form.