There is a distressingly long list of battles where I have had family on both sides. The earliest that I know of for sure was Agincourt, 25 October 1415, and the most recent was Antietam, September 17, 1862. All of the major battles of the War of the Roses fall into this category.
WWG1WGA
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)