Final answer:
The United States gained the most from the entry of Texas into the United States.
Step-by-step explanation:
The group or country that gained the most from the entry of Texas into the United States was the United States itself. By annexing Texas, the United States expanded its territory and gained access to valuable natural resources, like land and mineral wealth. Additionally, the annexation of Texas served the interests of expansionists in the slaveholding South, who believed in the destiny of slavery.