The Hidden Target — Helen MacInnes

Chance is the trigger that unleashes this whirlwind adventure. It unexpectedly brings Nina O'Connell and Robert Renwick face to face on a sunlit street in Amsterdam. Their last meeting had been in Geneva six years before, a time when Nina, fifteen thought herself in love with this American army major attached to NATO. The years, Nina notes, have made Bob even more attractive; the years, Renwick notes, have transformed that pretty, enchanting girl into a beautiful, poised yound woman.
In a nearby cafe they exchange news: Nina is off on a lark, an expense-free trip for a small group of students across Europe, the Near East, India, and, finally, the United States. Renwick is leaving NATO and the the army to join a company of consulting engineers. Neither description is accurate.
Renwick, in charge of NATO's antiterrorits section, is setting up an international intelligence agency, operating under cover and concentrating on the bloody, crazed epidemic of terrorism now running rampant. Nina's tour is not the lark she imagines. The two young leaders of the group are, in fact, expertly trained, experienced terrorists, part of a well-financed, brilliantly organized, meticulously concealed structure. There is a final objective, sinister and devastating - and Nina had been chosen for the part she, in all innocence, will play.