It isn't a battlefield but one could argue that the birth of the Republican Party had a ton to do with the start of the Civil War. Sure slavery, and all its side issues like states rights, was the primary cause but it wasn't until the Republican Party came along that things reached a boiling point.
Ripon, Wisconsin is one of four cities that claim the title of birthplace. Jackson, Michigan is where the first meeting was held that called themselves Republicans, but I've never been to Jackson. I have been to Ripon.
Fifty three men met in this little schoolhouse on March 20, 1854 to discuss the passage of the Kansas-Nebraska Bill. The men represented a bunch of different political parties; Whigs, Democrats and Free Soilers, but emerged united as a new group. Later this group would call themselves Republicans. Six years later they had elected Abraham Lincoln as President.
The small school house remains with a tiny gift shop inside (its basically one display case). A marker and a monument are next to the building. Being there it is interesting to reflect on how a party born here (or at three other small town meetings) grew into a major political party in six years. Also, not to belittle Ripon, but it is a small town and the thought of any major political force coming out of a meeting in such a small building in such a small town seems odd. Why wasn't the birth of the party in a town like Milwaukee or Madison? Or in a big city in New England? But on the other hand it is neat to see that a small grassroots group could effect big change.