The annexation of Texas was one of the most momentous actions the United States government took in the antebellum period. Apart from adding what was the largest state in the Union at that time, it expedited further avenues for westward expansion, exacerbated tensions with Mexico resulting in the Mexican-American War, and accelerated the sectional conflict over slavery. While the familiar concept of Manifest Destiny gives the impression that Texas joining the United States was inevitable, the history is much more complicated. In Adding the Lone Star, Jordan Cash explores how the decisions and actions of a cast of political actors in the United States, Texas, Mexico, and Great Britain contributed to the addition of Texas to the Union. Cash focuses on the annexation of Texas as a two-president decision while examining the administrations of American President John Tyler and Texian President Sam Houston, providing a comparative case study of the American and Texian presidencies to better comprehend how executive authority may be used in a system of separation of powers. Tyler’s ability to push his agenda on Texas despite the lack of institutional support shows the strength of premodern presidential power. Houston’s actions give an alternative view of executive authority since the Texian Republic, including the powers bestowed on the presidency, was structured on the model of its American counterpart. Tyler viewed the decision to annex Texas as beneficial for the United States as a whole while Houston considered it to be beneficial for Texas and proponents of slavery; Tyler’s secretary of state, John C. Calhoun, saw the decision as a victory for the South and the expansion of slavery. The examination of how these two presidents worked on the same issue at the same time but in largely different constitutional, institutional, political, and geographical contexts provides not only a better understanding of the history and politics of annexation but also an investigation of the nuances of presidential power in a constitutional system of checks and balances and separation of powers.