A Promise (2013)

Young Friederich has humble origins, but rises to the attention of his new boss, Karl. As he volunteers to tutor his employer's son, he gets more and more attached to Karl's young wife Lotte. She refuses however to betray her husband even when they learn Friedrich must go to Mexico for two years to supervise a mining project for Karl. Friedrich and Lotte swear to one another they will stay true to each other, but the oncoming war keeps them apart for far longer than expected.