The Allgaier Gambit is a dangerous, but theoretically dubious, knight sacrifice that begins with 1.e4 e5 2.f4 exf4 3.Nf3 g5 4.h4 g4 5.Ng5 (instead 5.Ne5 is the sounder and more conservative Kieseritzky Gambit). The knight on g5 has no safe retreat squares, and after 5...p, White plays 6.Nxf7 Kxf7, to force the black king out into the open.
Thanks to the long-term Attacking chances that White gets against the black king, the Allgaier Gambit produces many fine, Attacking wins in practice, but White risks ending up a piece down for inadequate compensation against prepared opponents, especially in correspondence play. The Hamppe-Allgaier Gambit, with Nc3 and ...Nc6 inserted, offers White better chances than the "pure" Allgaier.
Johann Baptist Allgaier (June 19, 1763, Schussenried – January 3, 1823, Vienna) was a German-Austrian chess master and theoretician. He was also the author of the first chess handbook in German – Neue theoretisch-praktische Anweisung zum Schachspiel (Vienna 1795–96).
King's Gambit Accepted, Allgaier and Kieseritsky Gambits including Rice Gambit