Final answer:
Sam Houston held major political positions as the governor of Tennessee, commander of the Texan military, and the first president of the Republic of Texas, where he led Texas's annexation to the U.S.
Step-by-step explanation:
The major political positions held by Sam Houston included serving as the governor of Tennessee, commanding the small military force of the Texan Independence Party, and notably, being elected as the first president of the Lone Star Republic, also known as the Republic of Texas, after gaining independence from Mexico. His leadership was significant during the Texas Revolution, where he secured victory at the Battle of San Jacinto, leading to the capture of Santa Anna and the treaties of Velasco. Later, as president of the independent Texas, Houston worked closely with U.S. President James K. Polk to annex Texas to the United States in 1845, a move that escalated tensions over slavery and territorial disputes which would lead to the Mexican-American War.