After a nearly four-year absence, professional soccer will return to RFK Stadium today with the first two games of the inaugural Ambassador Cup tournament.

The two-day, four-game tournament will feature a third incarnation of the Washington Diplomats and the Cosmopolitan Eagles -- both members of the newly formed American Soccer League -- the U.S. under-20 national team and the national team of Honduras.

In tonight's matches, the Diplomats will play the U.S. under-20 team -- which will play in the world youth championships in October in Chile -- at 7. The Eagles will play Honduras at 9.

Sunday, the Eagles will play the U.S. under-20 team at 4 p.m. and the Diplomats will play Honduras at 6.

The champion will be determined on the traditional soccer points system: two points for a win, one for a tie. In case of a tie, goal differential will determine the winner.

The Diplomats will have many players currently with Club Espana, one of four teams still in contention for the U.S. Soccer Federation men's open national championship.

Among the Club Espana players who will be wearing Diplomats uniforms this weekend will be goalkeeper Steve Pfeil, defenders Keith Trehy and Ronnie Simmons, midfielders Richard Burke and Duncan Reynard and forwards Fernando Iturbe and Daniel Cabrera.

The Eagles, based in Paterson, N.J., last weekend became the first ASL team to play a game, losing at home, 2-0, to Millonarios of Bogota, Colombia.

The ASL, which will also include F.C. Washington, a team that will be based at George Mason, is scheduled to begin regular-season play in April 1988 with at least six, and possibly eight, franchises.