Final answer:
James Long was the explorer who landed in Texas and built Fort St. Louis to establish their claim of Texas.
Step-by-step explanation:
The explorer who landed in Texas and built Fort St. Louis to establish their claim of Texas was James Long. In 1819, Long led an expedition across the Sabine River and succeeded in capturing Nacogdoches, writing a Declaration of Independence, and setting up a republican government. However, Spanish troops drove them out a month later.