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" 22d MAU Marines of a BLT 2/6 patrol, rifles at the ready and rounds chambered, take cover behind construction equipment on the coastal road south of Beirut after having been attacked on 16 March 1983.
The white-helmeted soldier and the one with dark glasses are soldiers belonging to the French contingent, United Nations Interim Force in Lebanon (UNIFIL) . They happened to be passing by at the time of the incident."
The helmet in all probability is painted a light blue, indistinguishable from white in a B&W picture.
UNIFIL had no troops per se in based in Beirut, there was as far as I recall an office at the Docks and a small Accommodation block for visiting UNIFIL troops, in general UN troops were advised not to take leave passes R&R in Beirut, it was too volatile.
A good read with regards the series of pictures is here - https://www.usmcu.edu/Portals/218/US Marines In Lebanon 1982-1984 PCN 19000309800.pdf