As Britain negotiates its exit from the European Union, the difficulties of forging a limited connection have become evident. In the nineteenth century, the American Civil War provides the most dramatic evidence of failed disunion, but an earlier, quieter disunion – that of the Mormons in 1846 – also merits our attention. Since Joseph Smith founded the Church of Jesus Christ of Latter-Day Saints in 1830, Americans had viewed its adherents with suspicion; faced with persecution, Mormons migrated westward seeking to freely practise their religion. However, after an anti-Mormon mob murdered Smith in 1844, the church leadership decided to leave the hostile United States entirely. In 1846, Mormons began their disunion, leaving for neighbouring Mexico where they settled around the Great Salt Lake Basin; with the majority of Mexicans living thousands of kilometres away, freedom from interference seemed likely. However, two months later, the United States declared war on Mexico and, in the subsequent 1848 peace treaty, acquired the entire Southwest, including the region where the Mormons lived. They immediately tried to become a state, but Congress created Utah Territory instead. Thus, the Mormons had to negotiate this compelled reunion from a subordinate position. Nonetheless, they gained significant concessions, most notably the appointment of the head of the church as territorial governor. For a decade, they were confident about their ability to manage this reunion, but ultimately the United States demonstrated just how little actual power they had. When they pushed too far, the union pushed back, sending the army and revealing that the Mormons’ independence within it was purely illusory. The so-called Utah War ended with the transformation of Utah into a more typical territory, one with a traditional American separation of church and state. Despite the Mormons’ initial successes, they learnt that when a significant power differential exists, there are limits to enjoying the benefits of union while remaining at a remove.