How others will see it. Military historians will be interested in the battle scenes, some of which were filmed at the front. There are primitive tanks, dirigibles, and poison gas dispensers. However, trench warfare is unrealistically presented. The film makes it appear relatively easy for a unit to go from one trench to another. The actual odds of penetrating a fortified trench were quite low, which explains why the war was at a standstill for some four years.
The romantic subplots are strictly pedestrian. But classic movie buffs will enjoy the young Lillian Gish, even though her character is laughably stereotyped. Some young women wore bright halos in 1918, I suppose, but the feminine ideal has since evolved.
How I felt about it. Her sister, Dorothy Gish, has a more saucy character character, and is cinematically punished for it. She doesn't get her man, and has to settle for an unromantic comic relief clod. Fortunately for them, Griffith generously allows for both bad girls and clods to become heroes during wartime, because in his mind, they represent the people of oppressed France.
The film may begin as a romance, but soon it is all about the villainous Huns, who make dainty Lillian toil in the fields, and whip her (lightly) when she can't lift a bushel of produce. One of them even tries to sexually assault her, but is predictably thwarted.
Hearts of the World was a propaganda film, funded by the British government and designed to rally Americans to the cause of France. Although France had stirred up considerable trouble one century earlier, by 1914, they were the victims of German aggression, partly occupied by the enemy. American's isolationist tradition needed to be overcome, and D.W. Griffith was happy to oblige.
But once stripped of its moral lessons about how women should behave, how soldiers (exemplified by the Harron's heroics) should excel, and how invaders should be defeated, we are left with a love story between a couple caught in a perilous, tumultous environment. That true love perseveres despite all obstacles is the ultimate message of the film. Or at least, D.W. Griffith believed that was what audiences wanted to see. There's no "Romeo and Juliet" tragedies here.
Future film directors Erich von Stoheim and Noel Coward are extras. You are unlikely to spot them.