The Toledo War. I believe it was the last armed conflict between states (or in this case, a state and a territory) outside the Civil War. Ohio and Michigan both claimed the strip of land that included Toledo and the mouth of the Maumee River, which was at the time a critical juncture in the shipping of goods from the Midwest to the East using the Great Lakes and a series of canals, which was far more cost effective then the old method of shipping goods down the Mississippi, and then along the Gulf Coast and up the entire Atlantic seaboard.
Despite the fact that Ohio had long claimed the land, it was effectively cut off from the rest of the state by the Great Black Swamp, so Michigan Territory occupied it and began setting up government services. Ohio protested, and tried to enforce its own laws there. Both states raised militias in preparation for war, but the only casualty came when a Michigan sheriff tried to arrest an Ohio citizen, Major Benjamin Stickney, who resisted. Stickney's son, Two*, stabbed the sheriff in the leg, and fled across the river to Ohio. The wound was not life threatening.
Congress resolved the matter by giving the Toledo Strip to Ohio, but giving Michigan the Upper Peninsula and statehood. Michigan rejected the compromise at first, but eventually accepted it. It was considered a bad deal for Michigan at first, but later, as railroads and the increasing size of cargo ships diminished Toledo's economic value, and valuable natural resources were discovered in the UP, it was generally agreed that Ohio got the short end of the stick in the long run.
en.wikipedia.org
* So named because he was Stickney's second son, no joke. His older brother was One Stickney. His sister, named after the state the Major had served in as an Indian agent, was Indiana Stickney.