What is true about Texas's entry into the United States?

The statement that Texas entered the United States via treaty is accurate. When Texas, which had declared independence from Mexico in 1836, sought to join the United States, its annexation was formalized through a treaty. This treaty, however, was controversial and sparked debates within the U.S. regarding the potential expansion of slavery and relationships with Mexico. Ultimately, Texas's annexation was approved by a joint resolution of Congress rather than a formal treaty, but it is often remembered as a treaty due to the historical context of Texas's unique status as an independent republic before its annexation.

In contrast, other options do not accurately describe the circumstances surrounding Texas's entry into the United States. Texas was not annexed through a standard territorial acquisition process, nor was it the first state to join the union (that distinction belongs to Delaware). Additionally, Texas did not join as part of a larger purchase; rather, it was an independent republic that negotiated its entry into the union on its own accord.