This is the most powerful old anti-war song that I know, but in a good earlier one a soldier (returned from Flanders, c 1692) tries to convince his younger brother to go to war. It's hard to refute the younger brother's objections. This is "The Farmer's Son of Devonshire", and preceeds "Will you go to Flanders" in my Scarce Songs 2 file. A few earlier anti-war songs are also known, and Martin Parker's "The Maunding Soldier" (see my broadside ballad index) illustrates the old adage 'The fruits of war is beggary'.
Incidently, I don't think one should make too much of the woman's name. There are some copies of the tune in late 18th century American manuscripts, where titles are "Will you go to Flanders, Molly" and "Will you go to Flanders, Jane". [Well, compilers of music manuscripts often couldn't spell either, and in the Giles Gibb's MS of 1777, the tune "Yankee doodle" is called "Thehos Gender", and I don't think that one has been figured out yet. My best guess is "The Horse Grenadier", but I wouldn't bet much on it.]