Shoulder Arms was Charlie Chaplin's final contribution to the World War I effort, along with his personal appearances selling Liberty Bonds and his film The Bond. It was released shortly before the end of the war, and Chaplin made prints available to soldiers fighting overseas, for which he was lauded for cheering the severely tested troops.
Charlie is a member of the Awkward Squad and we first see him being put through his paces in training camp. He has problems with making a proper about-face and with marching, his out-turned feet, constantly annoying his drill sergeant. Exhausted after a hard drill, he collapses on his cot.
Over there, somewhere in France, the troops are engaged in trench warfare, and Chaplin gives the audience a hilarious view on the difficulties experienced by the troops -- flooded quarters (which he shares with a sergeant played by brother Sydney Chaplin), constant shelling, sniping and homesickness.
In a touching scene, a mail-less Charlie reads a letter from home over the shoulder of another soldier and on his face we can see his emotional reactions to the good and bad news that the soldier reads.
Charlie is sent over the top and ends up capturing a squad of German soldiers single-handedly. His next foray, in the guise of a tree, provides a wonderful look at Chaplin's pantomime talents as he becomes a tree each time the enemy soldiers approach. Escaping the enemy squad he hides in a bombed-out house where a French girl, Edna Purviance, lives.
She discovers him in her bed and tends to his wounds. Soon they're beset by the enemy squad, searching for Charlie. In the chase, they collapse the rickety house and Charlie escapes, but Edna is arrested for aiding the enemy. Meanwhile Charlie's sergeant buddy is captured while attempting to telegraph information on the enemy to the allied camp.
Edna and Sydney are both brought to the enemy headquarters and Edna is threatened by the evil commandant. Charlie, sneaking down the chimney of the commandant's house, rescues Edna from his advances and locks him in a closet. At that moment the Kaiser, Crown Prince and their General arrive at the camp.
Charlie, rushing to the closet, takes the commandant's uniform and impersonates him. Taking charge of Edna and escorting her outside, he is recognized by his captive buddy, and the three of them overcome and restrain the Kaiser's driver and guards and replace them.
When the Kaiser and the others enter the limousine, the allies drive them off to the American camp, where Charlie is hailed as a hero and is hoisted on the shoulders of his comrades. But it was all a dream - in classic Chaplinesque-style Charlie is shaken awake by his drill sergeant -- still in boot camp!