The King's Gambit is a chess opening that begins with the moves: White offers a pawn to divert the black e-pawn.

The King's Gambit was one of the most popular openings for over 300 years, and has been played by many of the strongest players in many of the greatest brilliancies, including the Immortal Game.

François-André Danican Philidor (1726–1795), the greatest player and theorist of his day, wrote that the King's Gambit should end in a draw with best play by both sides, stating that "a gambit equally well attacked and defended is never a decisive [game], either on one side or the other.

"[3] Writing over 150 years later, Siegbert Tarrasch, one of the world's strongest players in the late 19th and early 20th centuries, pronounced the opening "a decisive mistake" and wrote that "it is almost madness to play the King's Gambit.

After World War II, 1.e4 openings became more popular again, with David Bronstein being the first grandmaster in decades to use the King's Gambit in serious play.

Spassky did beat many strong players with it, however, including Bobby Fischer,[8] Zsuzsa Polgar,[9] and a famous brilliancy against Bronstein himself.

[10] In 2012, an April Fools' Day prank by Chessbase in association with Vasik Rajlich—author of chess engine Rybka—claimed to have proven to a 99.99999999% certainty that the King's Gambit is at best a draw for White, but only after 3.Be2.

[14] The main reason is that it is hard to gain an opening advantage with White against strong opponents, with GM Matthew Sadler once joking that the dream of every King's Gambit player is a "worse but holdable ending".

[15] A handful of grandmasters have continued to use it, including Joseph Gallagher, Hikaru Nakamura, Baskaran Adhiban, Nigel Short, and Alexei Fedorov, albeit never as a main weapon.

Although Black usually accepts the gambit pawn, two methods of declining the gambit—the Classical Defense (2...Bc5) and the Falkbeer Countergambit (2...d5)—are also popular.

[17] With 4.h4 White practically forces 4...g4, thereby undermining any attempt by Black to set up a stable pawn chain with ...h6 and ...Bg7.

Boris Spassky used it to beat Bobby Fischer in a famous game at Mar del Plata in 1960.

6.Bc4 Rh7 (or 6...Nh6) is considered old-fashioned and risky, as Black loses a lot of time attempting to hold on to the pawn.

[20] The extremely sharp Muzio Gambit[21] arises after 4.Bc4 g4 5.0-0 gxf3 6.Qxf3, where White has sacrificed a knight but has three pieces bearing down on f7.

The Quaade Gambit (3.Nf3 g5 4.Nc3) is named after a Danish amateur who discussed it in correspondence with the Deutsche Schachzeitung in the 1880s.

[25] The move has received renewed attention following its recommendation by John Shaw in his 2013 book on the King's Gambit.

The Quaade Gambit has recently been advocated by Daniel King in his PowerPlay series for Chessbase.

It was played by Mark Bluvshtein to defeat former world title finalist Nigel Short at Montreal 2007,[31] even though it has never been highly regarded by theory.

Black concentrates on gaining piece play and fighting for the initiative rather than keeping the extra pawn.

It has been recommended by several publications as an easy way to equalize, although White's extra central pawn and piece activity gives a slight advantage.

This variation was considered most critical in the past, but recent trends seem to indicate a slight advantage for White.

Fischer then decided to refute the King's Gambit, and the next year the American Chess Quarterly published his analysis of 3...d6, which he called "a high-class waiting move".

The resulting slightly odd position (diagram) offers White good attacking chances.

John Shaw writes: "If given the time, Black intends to seal up the kingside with ...h4 followed by ...g5, securing the extra pawn on f4 without allowing an undermining h2–h4.

?, Maurian Defense, is relatively untested, but if White plays 4.Nf3 Black can transpose into the Hanstein Gambit after 4...g5 5.d4 Bg7 6.c3 d6 7.0-0 h6 (Neil McDonald, 1998).

It was once considered good for Black and scored well, but White obtains some advantage with the response 4.d3!, and the line fell out of favor after the 1930s.

The main line continues 4.Nc3 exf4 5.Nf3 Bd6 6.d4 Ne7 7.dxc6 Nbxc6, giving positions analogous to the Modern Variation of the gambit accepted.

5.d4 Nxf3+ 6.Qxf3 Qh4+ 7.Qf2 Qxf2+ 8.Kxf2 with a small endgame advantage, as played in the 1968 game between Bobby Fischer and Bob Wade in Vinkovci.

In several lines of the Vienna Game White offers a sort of delayed King's Gambit.

Both of these lines may be reached via the King's Gambit proper, but the Vienna move order is more common.