The marines call it the ''basic Beirut Christmas.'' There's a small fir tree and a red-and-gold sign proclaiming ''Season's Greetings.''
One day last week, Marine Cpl. Darrell Banks chopped down a small fir tree near Kilo Company's command post on the edge of Beirut airport and planted it in his tent in 2nd Platoon, about 100 yards behind the main runway of the international airport.
There will be turkey and trimmings in the cafeteria of Middle East Airlines today (Christmas Eve), carol-singing led by a group of Marines who will tour the airport area in a flat-bed truck, and midnight religious services. There are 1, 800 United States Marines serving in Beirut.
In the Sinai desert, near the North Camp of the Multinational Force and Observers, two Christmas trees are growing. They belong to the 2,000 soldiers of 11 countries stationed here as guardians of the 1979 Egyptian-Israeli peace treaty.
The two Jerusalem pines were transplanted two months ago from a grove planted by the Israeli Army before it turned over what was once the Eitam airbase to Egypt and the multinational force last April.
There are about 350 Americans among the 2,000 soldiers at North Camp. The rest of the American contingent, a battalion of the 101 Airborne Division, is based at South Camp at Sharm el Sheikh about 200 miles to the south, at the Sinai's southern tip.
Many of the soldiers will get leave to visit Jerusalem and Bethlehem - about a four-hour drive from North Camp.