Prince William -- The British Are Coming! A Lot of Them ... (VIDEO)

Prince William arrived at Reagan National Airport Monday morning as part of his three-day jaunt in the States ... and he rolled with a HUGE entourage of Brits.

Our photog tried to ask William a question -- and he did flash a smile and wave -- but getting any closer would probably have been met with resistance from the gaggle of suited security guards by his royal side.