Final answer:
The likely candidate for American annexation in the 19th century after Texas was the Oregon Country.
Step-by-step explanation:
The likely candidate for American annexation in the 19th century after Texas was the Oregon Country. This territory, comprising present-day Oregon, Washington, Idaho, and British Columbia, was a major area of expansion for the United States. Both Britain and the U.S. claimed the Oregon Territory, but eventually, the U.S. settled the matter peacefully through negotiations with Great Britain.