Sound of the Trumpet — Grace Livingston Hill

The first time beautiful, wealthy Lisle Kingsley sees John Sargent, she knows she loves him. But John is all too aware of the differences in their social standings. Besides, Lisle is practically engaged to wealthy Victor Vandingham, whose father is in charge of manufacturing a top secret weapon that supposedly will bring the war to a rapid and decisive end.

Then John is approached by men who are scheming to steal the weapon and its blueprints... men who know of John's growing friendship with Lisle... men who will stop at nothing--even kidnapping Lisle--to get John to cooperate with them!