The Good-Bye Kiss(1928)

Sally (Sally Eilers) joins the Salvation Army in order to follow her boyfriend Johnny (Johnny Burke) when he's shipped off to fight in the First World War. When Johnny turns out to be a bit of a coward, Sally spurs him and his fellow soldiers on to heroism. The Good-Bye Kiss was noted at the time for being a Mack Sennett comedy played relatively straight, with plenty of plot and very little slapstick.